11^th International Fermi Symposium - Talk Abstracts

Moaz Abdelmaguid (New York University/New York University Abu Dhabi); Samayra Straal, Joseph Gelfand, Jordan Eagle

Multi-wavelength Modelling of the Pulsar Wind Nebula Kes 75

The Kes 75 pulsar-wind nebula (PWN) is a captivating system due to its multifaceted nature. At its core is PSR J1846-0258, a young and energetic rotation-powered pulsar (RPP) with a very strong dipole magnetic field strength (B_NS = 5 * 10^13 G) that displays magnetar-like X-ray outbursts, flux variability and timing irregularities. Investigating this system will thus shed some light on the evolution of neutron stars by inferring the birth properties and nature of the stellar progenitor, supernova explosion, and the resultant neutron star. Currently, the best way of doing this is by using a one-zone evolutionary model of a PWN inside an SNR to reproduce its broadband spectral and dynamical properties. By modelling the broadband emission of this source -incorporating the new Fermi-LAT data- we found that it requires a very hot background photon field, suggesting the presence of a Wolf-Rayet (WR) star embedded within the PWN, which supports the unusually low ejecta mass found for this source. This also further suggests the WR star was possibly the binary companion of Kes 75, before its explosion. In this talk, I will describe these results and discuss their implications.

Marco Ajello (Clemson University); Chris Karwin (NASA/GSFC), Kohta Murase (Penn State University); Fermi-LAT collaboration

MeV Studies of Hadronic Emission in Nearby Seyfert Galaxies

The recent detection of high-energy neutrinos by IceCube in the direction of the nearby Seyfert/starburst galaxy NGC 1068 shows that radio-quiet AGN can accelerate cosmic rays (CRs). The interaction of these CRs must happen in a region of the galaxy that is highly opaque to GeV-TeV gamma rays, otherwise the GeV-TeV emission would violate the constraints provided by Fermi-LAT and MAGIC. The high-energy emission is reprocessed to lower energies and can finally leave the source as MeV radiation. This makes the <200 MeV energy band an important regime to explore with Fermi-LAT. In this talk, I will report on the analysis of Seyfert galaxies at >20 MeV and discuss what we can learn about the hadronically generated component in the corona.

Jason Alford (NYU); Joseph Gelfand, Eric Gotthelf, Kaya Mori, Pat Slane

Cosmic Ray Leptons Escaping from CTA 1?

Many Galactic TeV sources are associated with middle-aged (~10^4-5) pulsar wind nebulae (PWNe), with the γ-ray emission extending beyond the X-rays. This suggests that high-energy $e^pm$ first escape from the PWN, and then produce the γ-rays through inverse Compton (IC) scattering. However, how they escape from the PWN is poorly understood, in part because there are very few examples of systems where the particles are 'caught in the act' of escaping from the PWN. Understanding this process requires spatially-resolved measurements of the diffuse, X-ray spectrum of the extended TeV source. Spatially resolved spectral measurements of the PWN in CTA 1 are suggestive of high-energy e^pm 'caught in the act' of escaping from the PWN. We have analyzed a new 150 ks NuSTAR observation of the PWN in CTA 1, as well as an archival 120 ks XMM observation, and will present initial results from our analyses.

Axel Arbet-Engels (Max Planck Institute for Physics); David Paneque, Lea Heckmann, Felix Schmuckermaier, Ioannis Liodakis; "MAGIC, Fermi-LAT and MWL collaborators

Results from the first combined VHE and X-ray polarization measurements of TeV blazars

In 2022, the IXPE satellite provided the first measurement of X-ray polarization in TeV blazars, which opened a new window for testing acceleration and radiation models in AGNs. In this contribution, we explore the X-ray polarization evolution of the two archetypical TeV blazars Mrk421 and Mrk501, and combine it, for the first time, with simultaneous data from the radio up to very high energy (VHE; >0.1TeV). The VHE observations were performed by the MAGIC telescopes, and we additionally organized long observations with the XMM-Newton and NuSTAR telescopes to precisely investigate variability patterns up to the hard X-rays on hourly timescales.

We find Mrk421 in a variety of emission states, allowing us to correlate different polarization signatures with the spectral and flux evolution. During two of the IXPE exposures, a rotation of the X-ray polarization angle is observed. Throughout the rotation, our NuSTAR analysis reveals spectral hysteresis patterns, in both clockwise and counterclockwise direction, providing important constraints on particle acceleration efficiency. During the IXPE campaigns, Mrk501 is characterized by a significant decrease of the synchrotron peak frequency, which is accompanied by a drop in the X-ray polarization. Building on the IXPE results indicating an energy-stratified jet, we employ a multiple-zone model to describe the radio-to-VHE spectrum. Our model explains quantitatively the multi-wavelength polarization trend and the shift of the synchrotron peak can be connected with the change in polarization by a change of magnetization and/or emission regions sizes.

Hugo Ayala (Pennsylvania State University); HAWC Collaboration

Gamma-ray Observations of the IC 443 region with HAWC

The Jellyfish Nebula, or IC443, is a source of gamma rays. We report our observations of this region using data from the HAWC observatory. Our results are consistent with previous very-high energy observations, showing a soft spectrum and no sign of a cutoff. Unexpectedly, we also find a second component in the emission at these high energies that is extended and shows a hard spectrum with no evidence of a drop in the spectrum. The emission overlaps with the 90 kyr pulsar B0611+22. We present evidence that this emission is a TeV Halo produced by this pulsar.

Elias Aydi (Michigan State University/Texas Tech University)

New insights into novae - a new class of Galactic particle accelerators established by Fermi

Novae are panchromatic transients triggered by a thermonuclear runaway on the surfaces of white dwarf stars in interacting binaries. Our understanding of how novae are powered has been altered with Fermi establishing novae as a new class of gamma-ray sources and particle accelerators in our Galactic backyard. This unexpected discovery underscores the complexity of novae and their value as nearby laboratories to study high-energy emission and particle acceleration in shocks. In this talk I will highlight our ongoing multi-wavelength/multi-messenger efforts aimed at understanding how shocks work in novae. These efforts can help us probe critical but poorly understood physical processes, such as common envelope interaction, super-Eddington luminosities, particle acceleration efficiency, and dust formation around explosive transients, and are essential for a better understanding of other, more distant, shock-powered transients in the Universe such as supernovae, stellar mergers, and tidal disruption events.

Rishi Babu (Michigan State University); HAWC Collaboration

HAWC Study of the extended TeV emission from HESS J1813-178

HESS J1813-178 is a TeV source potentially associated with one of the most powerful pulsars in the sky, PSR J1813-1749 and its pulsar wind nebula (PWN). With the age of the pulsar at ~5 kyrs, the gamma-rays from this region can be used to study the extreme environment around young pulsars. Previous studies by H.E.S.S. and MAGIC collaborations, show a point-like source in the region with emission extending beyond 10 TeV. Using 2139 days of HAWC data, we find two distinct emission regions, a compact source spatially coinciding with PSR J1813-1749 and an extended region spatially coinciding with a low-mass X-ray binary (LMXB) GX 13+1. To understand the nature of the extended emission, we analyze this region using 14 years of Fermi data in order to find flux modulation for the LMXB which has a period of 24 days. Using results from this analysis, we perform a multi-wavelength spectral modeling to explore the mechanisms behind the TeV gamma-rays.

Matthew Baring (Rice University); Hoa Dinh and Zorawar Wadiasingh

Comptonized Relativistic Winds in Magnetar Giant Flares

In April 2020, the Fermi Gamma-Ray Burst Monitor (GBM) detected the intense initial spike of an extragalactic magnetar giant flare (MGF) from NGC 253. The excellent time-resolved spectroscopy delivered by the unsaturated scintillators of Fermi-GBM for GRB 200415A provided an unprecedented view of such a flare, leading to the relativistic, rotating lighthouse interpretation of this MGF. This paper addresses this topical transient, modeling the relativistic wind expansion and its adiabatic cooling in the collimating polar field portion of the magnetosphere. Radiative transfer simulations of Thomson scattering are employed to describe the anisotropy and polarization of emission from the exterior flared surface of the relativistic wind. These simulations also help define the super-Eddington dynamics of the wind. The direction-dependent Doppler boosting and beaming of the soft gamma-ray signal leads to a sequential hardening and intensification followed by a softening and flux decline as the emission beam passes over an observer's line of sight. This spectral evolution can be used to probe the magnetar's rotation period and the wind's coasting Lorentz factor, and possibly also the level of twist in the magnetospheric field if it does not assume purely dipolar morphology.

Monica Barnard (University of Johannesburg, South Africa); Ankur Ghosh and Jagdish C. Joshi and Soebur Razzaque

Modeling multiwavelength afterglows detected from the VHE-GRB population with NAIMA

Gamma-ray bursts (GRBs) are extremely bright sources observable across the entire electromagnetic spectrum. Until now Fermi-LAT detected GRB afterglow emission up to GeV energies, although theories predicted that this late time emission could extend to even higher energies. Recent detections by ground-based telescopes confirmed this when afterglow emission in the very-high-energy (VHE, $E\geq100$ GeV) range was reported. This VHE emission has been extensively modeled as synchrotron-self-Compton (SSC), assuming both the stellar wind and constant density circumburst scenarios. Other studies considered hadronic emission mechanisms such as photo-pion and photo-pair processes, and proton synchrotron. We interpret the VHE emission as a combination of external Compton (EC) and SSC radiation, in both the stellar wind and constant density environments. We use a publicly available NAIMA radiation modeling code to study the afterglow emission from several VHE-GRBs, and present multiwavelength light curves and energy spectra. For emission at these VHE the $\gamma$-ray attenuation by absorption of photons through their interaction with the extragalactic background light becomes important and we have corrected the EC and SSC models accordingly. NAIMA already includes an optimization tool to fit the emission model to multiwavelength data, thus providing better constraints on the particle acceleration, emission mechanisms as well as the current models of GRBs at these late times.

Rodolfo Barniol Duran (California State University Sacramento); Michail Damoulakis, Dimitrios Giannios

The striped jet model: From the central engine to gamma-rays in GRBs

In the striped jet model, a magnetically dominated GRB (Gamma-ray Burst) jet has small-scale field reversals or "stripes", due to the field's non-axisymmetry at the central engine. Magnetic reconnection drives dissipation in the stripes, which causes jet bulk acceleration and particle energization out to a distance where most of the magnetic energy has been used up and the acceleration ceases. This model gives a robust prediction on the relation of jet acceleration and magnetization and their dependence as a function of distance. We use early afterglow modeling results to infer the values of the bulk Lorentz factor and magnetization at the deceleration radius (large distances from the central engine) and use these values as outer boundary conditions for the striped jet model. This allows us then to move closer to the central engine and infer valuable information on its nature (neutron star or black hole). We calculate the location where most of the energy is dissipated, compare it to the Thomson photosphere and calculate the expected gamma-ray signal, which is consistent with the majority of available prompt-phase GRB observations. We will describe recent Monte Carlo calculations within the context of the striped jet model and preliminary results.

Adrien Laviro (LLR, Ecole Polytechnique & CNRS / IN2P3 & IPP); Denis Bernard

The performance of silicon-pixel-based trackers for the polarimetry of gamma-rays converting to e+e- pairs

The trackers of present (The Fermi LAT) and project (AMEGO, ASTROGAM) spatial gamma-ray telescopes consist of mille-feuilles of silicon-strip detectors (SSD). I studied the performance of such devices for the polarimetry of gamma-rays converting to e+e- pairs in Nucl.Instrum.Meth.A 1042 (2022) 167462. Recently the new concept of trackers consisting of pixelated silicon sensors has been put forward (J.Astron.Telesc.Instrum.Syst. 8 (2022) 4, 044003), with the goal of lowering the capacitance of each segment and so of improving on the detection of low-energy Compton-scattering events. Using similar tools as for my past study, I characterize the performance of silicon-pixel-based trackers for the polarimetry of gamma-rays converting to e+e- pairs.

Joanna Berteaud (UMD/NASA GSFC)

Multiwavelength identification of millisecond pulsar candidates in the Galactic bulge

The existence of a population of millisecond pulsars (MSPs) in the Galactic bulge is supported, along with other evidence, by the Fermi GeV excess, an anomalous diffuse gamma-ray emission detected almost 15 years ago in the direction of the Galactic center. However, surveys searching for radio pulsations have not yet revealed bulge MSPs. These surveys are often shallow, and identifying promising bulge MSP candidates is key to motivating deep targeted pulsation searches. MSP candidates are usually selected among steep-spectrum or polarized radio sources, but multiwavelength information can also be exploited. In this talk, I will present our recent investigation of the multiwavelength counterparts of sources detected by the Chandra X-ray observatory that have spectral properties expected for MSPs in the Galactic bulge. We identified a large population of more than a thousand X-ray sources without optical, ultraviolet, or strong infrared counterparts. Among them, five are seen for the first time in unpublished radio imaging data from the Very Large Array, and we are currently conducting follow-up searches for radio pulsation toward these promising sources.

Markus Boettcher (North-West University); Anton Dmytriiev

Effects of non-continuous inverse-Compton cooling in blazars

Most numerical codes for modeling blazar emission utilize a simplified continuous-loss description of particle cooling due to inverse Compton (IC) scattering, neglecting non-continuous effects that arise in the Klein-Nishina (KN) regime. In this study, we investigate the importance of non-continuous Compton cooling losses and their impact on the electron spectrum and spectral energy distribution (SED) of blazars during high flux states (flares), as well as in the low state. We solve numerically the full transport equation accounting for large relative jumps in energy, by extending our existing blazar flare modeling code EMBLEM. We simulate the time-dependent electron spectra and SEDs, with observational characteristics similar to the brightest gamma-ray flare of the archetypal Flat Spectrum Radio Quasar (FSRQ) 3C 279. We then compare results obtained using the full cooling term with the continuous-loss approximation. We show that during flaring states of FSRQs characterized by high Compton dominance, non-continuous cooling can lead to a significant modification of the electron spectrum, introducing a range of distinct features, such as hardening/softening, narrow pile-ups and low-energy tails. Such distortion translates to differences in the associated SED up to ~ 40 %. This highlights the need to consider non-continuous effects in blazar emission models, particularly applied to extreme gamma-ray flares.

Michael Briggs (University of Alabama in Huntsville); O.J. Roberts, S. Lesage, M.Godwin, B. Mailyan, and R. Holzworth

The 3rd Fermi-GBM Terrestrial Gamma-ray Flash Catalog

We present the third Fermi GBM catalog of Terrestrial Gamma-ray Flashes (TGFs), containing 15 years of data. This work builds upon the previous GBM catalogs, with improvements to the off-line search and cosmic-ray rejection techniques. The search algorithm of the GBM Continuous Time-Tagged Event (CTTE) data is revised to find TGFs for which there is only signal in one GBM BGO detector, resulting in a weaker, second category of TGFs detected by GBM. We discuss these two categories of GBM TGFs, present new methods for measuring their temporal profiles, including the number of pulses per TGF, their spectral behavior, geographical distribution, and other TGF parameters. Finally, we will present a complete list of TGFs that are accompanied by a very-low frequency radio "sferic" from the World-Wide Lightning Location Network, reporting the sferic energies and locations. This third catalog will provide the atmospheric electricity and TGF communities with the largest, comprehensive TGF sample available.

Ettore Bronzini (University of Bologna & INAF-OAS); E. Torresi, S. Buson, P. Grandi, et al.; Fermi-LAT

Fermi-LAT analysis of the CSO NGC 4278 detected by LHAASO

Compact symmetric objects (CSOs) are sources with radio lobe emission on both sides of an active nucleus and an overall size of less than one kpc. From the detection of 3 CSOs by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope, we know that the emission from these objects can extend into the GeV band. Surprisingly, the first LHAASO catalog reported a TeV source, 1LHAASO J1219+2915, detected up to 25 TeV and tentatively associated with the CSO NGC 4278. In this contribution, we present the analysis of the LAT data in the region of 1LHAASO J1219+2915 at the time of the LHAASO detection. Our analysis revealed evidence for a new point-like source, detected at a statistical significance of ~5 sigma, spatially consistent with the LHAASO detection and the radio position of NGC 4278. We observed a hard spectrum in the Fermi-LAT band, with two very high-energy (VHE) photons (~100 GeV) associated with NGC 4278 with a probability exceeding 99%. Our results provide further support to the association between the LHAASO source and the CSO NGC 4278, posing new challenges for our understanding of the physical processes acting in relativistic jets.

Alessandro Bruno (CUA & NASA/GSFC); J. M. Ryan, G. A. de Nolfo, I. G. Richardson, S. Dalla and A. N. Hutchinson

Long-Duration Gamma-Ray Flares and Solar Energetic Particles

Large solar eruptions are often associated with high-energy (>100 MeV), delayed gamma-ray emission extending up to tens of hours after the flare impulsive phase. Such long-duration gamma-ray flares (LDGRFs), previously identified with the Compton Gamma-Ray Observatory and the Solar Maximum Mission, have been routinely measured in recent years by the Fermi-LAT, which has provided a major contribution to the characterization of these events thanks to its superior observational capabilities. However, while this phenomenon is known to be caused by high-energy ions interacting with the solar atmosphere, the underlying dominant acceleration process remains under debate. Potential mechanisms include continuous, second-order stochastic acceleration of flare-particles injected and trapped within large coronal loops, and acceleration at coronal mass ejection (CME)-driven shocks with subsequent back-propagation toward the Sun. The widely-invoked CME-shock paradigm is supported by the statistical association of LDGRFs with relatively-fast CMEs and large Solar Energetic Particle (SEP) events, and the spatially extended gamma-ray emission observed for behind-the-limb events. On the other hand, the apparent relationship with CMEs and SEPs does not necessarily provide conclusive evidence for or against a causal connection, and an efficient back-precipitation from CME-driven-shock heights is strongly impeded by magnetic mirroring. The flare-loop model represents a natural alternative, explaining both the prolonged/delayed emission and its broad spatial extent, although a more widespread acceptance of this scenario is disfavored by the present difficulty to visualize large-scale coronal loops. The "flare vs. shock" debate is far from over and will continue to foster our understanding of SEP acceleration at the Sun.

Niccolo' Bucciantini (INAF - Osservatorio Astrofisico di Arcetri)

Pulsar Wind Nebulae: the present status and future prospects

Pulsar Wind Nebulae are among the most interesting high-energy astrophysical sources, and they provide us with a unique laboratory where relativistic processes can be studied and characterized in high details. I will review how, in the past years, thanks to a combination of high resolution X-ray imaging, gamma-ray observations, and state of the art numerical simulations, our understanding of these object has progressed. I will then discuss how present and future gamma-ray facilities, in synergy with X-ray, might help us to shed light on the still mysterious acceleration processes, and on the ultimate fate of those accelerated particles.

Sara Buson (DESY, Univ. of Wurzburg); S. Buson, L. Pfeiffer, S. Marchesi, G. Fichet de Clairfontaine, A. Azzollini, A. Tramacere, J. M. Zaballa, M. Lincetto; on behalf of the Fermi-LAT collaboration

Hadronic processes at work in 5BZB J0630-2406

Recent observations are shedding light on the important role that active galactic nuclei (AGN) play in the production of high-energy neutrinos. In this study, we focus on one object, 5BZB J0630-2406, which is among the blazars recently proposed as associated with neutrino emission during the first 7-yr IceCube observations. Modeling the quasi-simultaneous, broad-band spectral energy distribution, we explore various scenarios from purely leptonic to lepto-hadronic models, testing the inclusion of external photon fields. Despite being historically classified as a BL Lac, our study shows that 5BZB J0630-2406 belongs to the relatively rare sub-class of high-power flat-spectrum radio quasars (FSRQs, a.k.a. masquerading BL Lacs). Our results indicate that interactions between protons and external radiation fields can produce a neutrino flux that is within the reach of the IceCube detector. Furthermore, the X-ray spectrum evidences the imprint of hadronic processes related to very energetic protons.

Benoit Cerutti (CNRS & Université Grenoble Alpes); Enzo Figueiredo, Adrien Soudais, Guillaume Dubus

Gamma-ray pulsars: Comparison between global PIC simulations and the Fermi catalog

The wealth of observational data collected by Fermi in the GeV range show that pulsars accelerate particles to extreme energies with a disconcerting efficiency. This activity is connected to the dynamic of the magnetosphere, filled with a relativistic magnetized plasma of pairs emitting pulses of light modulated by the rotation of the star. Decoding the information contained in the gamma-ray lightcurve is an important step to understand how pulsars work. In this work, we present a new series of global 3D PIC simulations of an inclined pulsar magnetosphere. Synthetic pulse profiles in the synchrotron and inverse Compton channels are reconstructed with unprecedented resolution and accuracy to allow for a direct comparison with the third Fermi pulsar catalog. Gamma-ray emission at the base of the wind current sheet explains all of the observed features, including the mysterious Vela-like lightcurves and the pulse narrowing with increasing photon energy. We show that the radiative efficiency is limited by the reconnection rate. Lightcurve fitting with the PIC model shows an excess of pulsars with low magnetic obliquities, possibly due to magnetic alignment. We also present a new high-resolution simulation based on a hybrid force-free PIC model, which allows for the first time to approach a realistic scale separation of the weakest observed millisecond pulsars seen by Fermi. This work gives more confidence in the rescaling procedure most needed to perform global 3D simulations, and more generally a better understanding of such extreme astrophysical objects.

Sarah Chastain (University of New Mexico); A. J. van der Horst, G. E. Anderson, L. Rhodes, D. d'Antonio, M. E. Bell, R. P. Fender, P. J. Hancock, A. Horesh, C. Kouveliotou, K. P. Mooley, A. Rowlinson, S. D. Vergani, R. A. M. J. Wijers, P. A. Woudt; ThunderKAT

Constraints on Short Gamma-Ray Burst Physics and Their Host Galaxies from Systematic Radio Follow-up Campaigns

Short gamma-ray bursts (GRBs) are explosive transients caused by binary mergers of compact objects containing at least one neutron star. Multi-wavelength afterglow observations provide constraints on the physical parameters of the jet, its surrounding medium, and the microphysics of the enhanced magnetic fields and accelerated electrons in the blast wave at the front of the jet. The synchrotron radio emission can be tracked for much longer than in other spectral regimes, and it can pin down the evolution of the spectral peak. We present the results of a systematic observing campaign of eight short GRBs with the MeerKAT radio telescope. Additionally, we present observations of four of these short GRBs using the ATCA radio telescope and two of these short GRBs with the e-MERLIN radio telescope. Using these results we report one possible detection of a short GRB afterglow from GRB 230217A and deep upper limits for the rest of our short GRB observations. We use these observations to place constraints on some of the physical parameters, in particular those related to electron acceleration, the circumburst density, and gamma-ray energy efficiency. We discuss how deeper observations with new and upgraded telescopes should be able to determine if the gamma-ray efficiency differs between long and short GRBs. We also report detections of the likely host galaxies for four of the eight GRBs and upper limits for another GRB, increasing the number of detected host galaxies in the radio with implications for the star formation rate in these galaxies.

Songzhan Chen (Institute of High Energy Physics, Chinese Academy of Sciences); the LHAASO collaboration

Highlight results of LHAASO on extragalactic VHE gamma-ray sources

LHAASO is a large hybrid extensive air shower array being constructed at Haizi Mountain, 4410 m a.s.l., in China. It has an excellent sensitivity for gamma-rays with energies from sub-TeV 10 PeV and also has a large field of view that can continuously monitor a large fraction of sky with declination from -20 degree to 80 degree since 2020. In this talk, we will report the recent achieved highlight science results in extragalactic VHE gamma-ray source observations. These include: the first LHAASO gamma-ray source catalog yielded by the most sensitive sky survey at VHE and UHE, which include nearly one hundred gamma-ray sources with tens of new discovery; Important innovative observation of >10 TeV photons from GRB 221009A; Discovery of several flares from several AGNs, and so on. A prospect for the future observations and detector updating is also presented.

C.C. "Teddy" Cheung (Naval Research Lab); P. Jean, T.J. Johnson, P. Fauverge; Fermi-LAT collaboration

The population of gamma-ray emitting novae

We summarize the state of >100 MeV observations of Galactic novae based on more than 15 years of Fermi Large Area Telescope (LAT) observations. The disk and bulge populations are delineated using available distance measurements. Insights on gamma-ray emission mechanism based on studies of exemplary systems, particularly nearby symbiotic recurrent novae, will be discussed.

Cecilia Chirenti (UMD); Simone Dichiara (PennState), Amy Lien (University of Tampa), Cole Miller (UMD))

Evidence for a strong 19.5 Hz gamma-ray flux oscillation from GRB 211211A

The gamma-ray burst (GRB) GRB 211211A is believed to have occurred due to the merger of two neutron stars or a neutron star and a black hole, despite its duration of more than a minute. Here we perform timing analysis of Fermi GBM and Swift BAT gamma-ray data on GRB 211211A, and find a highly significant 19.5 Hz flux oscillation, which has higher fractional amplitude at higher energies, in a ~0.2 second segment beginning ~1.6 seconds after the start of the burst. After presenting our analysis we discuss possible mechanisms for the oscillation.

Antonio Circiello (Clemson University); A. McDaniel, C. Kharwin, M. Ajello, M. Di Mauro, A. Drlica-Wagner, M. Sánchez-Conde; Fermi-LAT

Constraining Dark Matter Annihilation with Fermi-LAT Observations of Ultra-Faint Compact Stellar Systems

Recent observations from optical surveys have discovered the presence of a multitude of ultra-faint compact stellar systems (UFCSs) orbiting the Milky Way (MW) that have the potential to be the most compact and faintest galaxies observed so far. If they were confirmed to be dark matter (DM) dominated, these objects would be ideal for indirect searches of DM annihilation, due to their proximity and relatively high DM content. We analyze 14.3 years of Fermi-LAT gamma-ray data coincident with 26 UFCSs, selected using the results from recent numerical simulations and models of galaxy formation. No excess gamma-ray emission is detected and we evaluate the gamma-ray flux upper limits for these systems. We derive the sensitivity for DM annihilation signal, assuming that these UFCSs are DM-dominated and consistent to the observed population of dwarf spheroidal satellite galaxies (dSphs) of the MW. We also account for the possibility that not all the targets in our sample are DM-dominated, by evaluating the sensitivity for random subsets of the selected UFCSs. This work shows the potential of the UFCSs to yield constraints on DM properties that are competitive with, if not improve, the ones obtained from dSphs, and highlights the importance of kinematic studies on these systems to empirically determine their DM content.

Milena Crnogorcevic (Oskar Klein Centre/Stockholm University)

Fermi Listens for WISPers: Past, Present, and Future of Fermi's Axion-like Particle Searches

Axion-like particles (ALPs), classified within the family of Weakly Interacting Sub-eV Particles (WISPs), represent a well-motivated candidate for constituting a significant fraction of dark matter. Given their expected signatures in MeV and GeV gamma rays, the *Fermi* Gamma-ray Space Telescope has been uniquely positioned to characterize ALP properties, providing some of the most stringent constraints on the ALP parameter space. I will delve into the pivotal role of *Fermi* over the past fifteen years in investigating ALP signatures emerging from a variety of astrophysical sources, including supernovae, gamma-ray bursts, active galactic nuclei, galaxy clusters, pulsars, and more. Looking forward, I will highlight the ongoing importance of *Fermi*'s observational capabilities, not only as a standalone observatory but also as a critical component of the current and future multimessenger infractructure.

Paolo Da Vela (OAS INAF Bologna); Peter Veres, Francesco Gabriele Saturni, Guillem Martì-Devesa, Antonio Stamerra, Manuel Meyer, Fancesco Longo, Lea Burmeister

Constraints on the intergalactic magnetic from Fermi-LAT observations of GRB 221009A

The observation of delayed GeV emission after a Gamma Ray Burst (GRB) detected at the very-high energies (VHE) beyond 100 GeV could indicate a non-zero magnetic field in the intergalactic medium. Indeed, VHE photons interact with the Extragalactic Background Light (EBL) to produce electron-positron pairs, which in turn can initiate electromagnetic cascades. An intergalactic magnetic field (IGMF) would deflect the pairs, leading to a delay of this emission, a so called pair echo. The VHE detection of GRB221009A with LHAASO at several TeV offers a unique opportunity to probe the IGMF. Here we use the reported LHAASO VHE spectrum and CRPropa Monte Carlo simulations to generate time and energy dependent predictions of the cascade for different IGMF strengths in the Fermi/LAT energy domain. Using these predictions, we search for the pair echo using the full Poisson likelihood information. Depending on the modeling of the astrophysical afterglow emission of the GRB, we are able to set new constraints on the IGMF strength, excluding fields below 1e-17 G

Sarah Dalessi (University of Alabama in Huntsville, Center for Space Plasma and Aeronomic Research); Stephen Lesage, Oliver J. Roberts; Fermi-GBM team

Fermi-GBM Observations of Recent Extraordinarily Bright Gamma-ray Bursts: GRB 221009A, GRB 230307A, and GRB 230812B

Gamma-ray bursts (GRBs) represent some of the most energetic explosions in the universe, emitting relativistic jets that last from milliseconds to minutes. Since 2022 there have been three GRBs observed by the Fermi Gamma-ray Burst Monitor (Fermi-GBM) that exhibit exceptionally luminous characteristics. All three of these bursts were so bright that they generated complex dead time and pulse pile-up effects in the GBM data. First was GRB 221009A, dubbed "The BOAT" (Brightest of All Time), which had a prompt emission episode lasting over 600s, a triggering pulse with emission up to ~15MeV, and boasts the highest total isotropic-equivalent energy of 1.0 x 10^55 erg and fluence of 0.21 erg/cm^2. Second was GRB 230307A which has the second highest fluence of observed GRBs with ~3 x 10^-3 erg/cm^2 and is a rare case of a long GRB with a compact merger origin which is generally associated with short GRBs. Lastly, GRB 230812B was a nearby, z=0.36, long burst with a fluence of ~3 x 10^-4 erg/cm^2 with an associated Type Ic-BL SN. I will discuss the diverse origins and emission episodes of these three distinctively bright GRBs as seen by Fermi-GBM.

Davide Depalo (Politecnico and INFN Bari); Elisabetta Bissaldi, Suman Bala, Adam Goldstein; Fermi-GBM Collaboration

Systematic time-resolved analysis of Gamma-Ray Bursts detected by Fermi-GBM

The Fermi Gamma-Ray Space Telescope has been operational for almost 16 years. During this period, the Fermi Gamma-Ray Burst Monitor (GBM) has been the most prolific Gamma-Ray Burst (GRB) detector ever, with more than 3700 observed GRBs to date. Tens of dedicated single-GRB publications and several general catalogs helped shedding light on the temporal and spectral characteristics of these fascinating events.

Here we present a systematic analysis of a subsample of bright GRBs, focusing on the evolution of the fitting model parameters during each event. Light Curves are binned using the Bayesian Blocks method, and the time-resolved analysis is performed by means of the newly developed GBM Data Tools. We applied five different spectral models to each time bin, and selected the best one through specific statistical criteria. We find that the best fit parameters distributions confirm the dominance of the Comptonized model. Finally, our goal is to implement the current pipeline for a future comprehensive and systematic analysis of a broader GRB sample.

Abhishek Desai (NASA GSFC (NPP)); Stefano Marchesi, Justin Vandenbroucke, Marco Ajello, Ke Fang, Dieter Hartmann, Regina Caputo, Sam Hori, Jessie Thwaites and Kavic Kumar; Fermi-LAT Collaboration; IceCube Collaboration

Multi-Messenger and Multi-Wavelength Studies with Active Galactic Nuclei

On 22 September 2017, IceCube reported a high-energy neutrino event, which was found to be coincident with a flaring blazar, TXS 0506+056. This multi-messenger observation, with IceCube neutrinos and gamma-rays detected by Fermi-LAT, hinted at blazars being sources of observed high-energy astrophysical neutrinos. However, a separate time-integrated study using IceCube data reported that a Seyfert galaxy, NGC 1068, is a source of astrophysical neutrinos. Because of the different nature of the two sources and the lack of significant neutrino and gamma-ray correlation, this causes confusion regarding the processes that lead to the production of these neutrinos in Active Galaxies. This issue can be tackled by more multi-messenger and multi-wavelength observations and relevant studies. Our work discusses a correlation study between neutrinos and photons from different wavelengths in AGN including data observed using the NICER, NuSTAR and Fermi-LAT telescopes and IceCube. We will also discuss the future implications of our work and how upcoming telescopes can help us better understand AGN processes.

Niccolo' Di Lalla (Stanford University); Nicola Omodei; Fermi-LAT Collaboration

Follow-up of Gravitational-Waves with Fermi-LAT and First Results from O4

A new era for astronomy began when the first detection of Gravitational Wave (GW) event arising from the coalescence of two stellar-mass black holes was detected by LIGO-Virgo in 2015. This groundbreaking discovery was then followed, in 2017, by the simultaneous observation of GRB170817A by Fermi-GBM and INTEGRAL, and the LIGO-Virgo detection of the binary neutron star merger GW170817.

Searches for electromagnetic counterparts of GW events are fundamental for obtaining important insights into the physics of compact object mergers and their emission components, and has now become a routine activity among all the space- and ground-based observatories. The Fermi-LAT has the best sensitivity to simultaneously observe a large fraction of the sky from about 30 MeV up to several hundred GeV, providing the unique ability to rapidly cover the entire probability region from a GW candidate. In this contribution, we will provide an overview of the LAT follow-up analysis pipelines and we will present the first preliminary results from the ongoing LIGO-Virgo-KAGRA observing run O4.

Alberto Dominguez (Universidad Complutense de Madrid/IPARCOS); P. Østergaard Kirkeberg, R. Wojtak, A. Saldana-Lopez, A. Desai, J. R. Primack, J. Finke, M. Ajello, P. G. Pérez-González, V. S. Paliya, D. Hartmann

Advancements in the Hubble Constant Estimation via Gamma-Ray Attenuation

The persistent discrepancy in the Hubble constant determinations, known as the Hubble tension, represents one of the most intriguing challenges in modern cosmology. For this reason, results from new and independent techniques are especially compelling. We will present the latest estimates on the Hubble constant (H0) from gamma-ray attenuation that uses optical depths calculations from a recent extragalactic background light model. This approach is built from multiwavelength galaxy data from the Hubble Space Telescope Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (HST/CANDELS). CANDELS gathers one of the deepest and most complete observations of stellar and dust emissions in galaxies. These optical depths are compared with data from gamma-ray observations taken with the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope and imaging atmospheric Cherenkov telescopes. We find H0 = 62.4 +4.1/-3.9 km s-1 Mpc-1 when fixing the matter density of the Universe, Ωm = 0.32, and H0 = 65.1 +6.0/-4.9 km s-1 Mpc-1 and Ωm = 0.19 ± 0.08, when exploring these two parameters simultaneously. Although with low significance, results from this methodology tend to align with Hubble determinations at cosmic scales such as those from baryonic acoustic oscillations and the cosmic microwave background rather than from local scales such as those from Cepheid variable stars and Type Ia supernovae.

Tulun Ergin (Michigan State University, Physics and Astronomy Department, MI, USA); Ramiro Torres-Escobedo, Tsung Dao Lee Institute, Department of Physics & Astronomy, Shanghai Jiao Tong University, Shanghai, China; HAWC Collaboration

Searching for HAWC Counterparts of "Dark" Sources Reported in the 1st LHAASO Source Catalog

Recent observations done in the energy range of 100 GeV - 100 TeV by the High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory and the Large High Altitude Air Shower Observatory (LHAASO) have located potential Galactic PeVatrons, thanks to their enhanced high energy flux sensitivities above 10 TeV. Candidate source classes for PeVatrons are supernova remnants, pulsar wind nebulae, star forming regions, massive star clusters, super bubbles, supernovae, and microquasars. However, the so-called "dark" gamma-ray sources are reported to lack associations to a defined source class in the 1st LHAASO Source CatalogABSTRACTABSTRACT", we want to show preliminary results of our search for the HAWC counterparts of these dark sources. In addition, we analyze Fermi-LAT data together with HAWC data," profiling spatial and spectral distributions of gamma rays from GeV to TeV energies.,

Abe Falcone (Pennsylvania State University); Steve Kerby, Kyle Neumann, Elizabeth Ferrara, Eric Grove, Jamie Kennea, Paul Ray, Michael Stroh

New Counterparts to Previously Unidentified Fermi Gamma-ray Sources

We have used Swift to search for X-ray and UV/optical counterparts of unassociated Fermi gamma-ray sources. This program has identified many new gamma-ray blazars, as well as some likely pulsars and a handful of difficult to classify sources. This Swift program includes pointed observations of hundreds of Fermi 4FGL catalog sources with no currently known source association at other wavelengths. For each of the Fermi-LAT localization ellipses, Swift-XRT obtains accurate source positions (typically < 5 arcsec) of any detected X-ray sources, enabling new blazar identification observations and pulsation searches at both radio and gamma-ray wavelengths. In many cases, UV and Optical Telescope observations have also been utilized to search for counterparts. Classification of these sources with neural network classifiers has resulted in many new blazar classifications, and sprectral energy distribution fitting has allowed us to explore these new, and typically dimmer, blazars to study their parameters and to explore whether or not they are an extension of the blazar sequence into the lower luminosity parameter space. The latest results and updated catalog of newly identified counterparts will be presented.

Paul Fauverge (LP2i Bordeaux CNRS - Université de Bordeaux); C.C. Cheung, P. Jean, K.V. Sokolovsky, J.D. Linford, K. Mukai, J.L. Sokoloski; Fermi-LAT collaboration

Recent Fermi novae in a multi-wavelength context

Two recent classical novae, V1723 Sco (2024) and V6598 Sgr (2023), were detected by the Fermi-LAT. V1723 Sco is one of the brightest novae observed to date by the LAT, providing a two-week window for detection. The extensive Fermi observations of V1723 Sco, complemented by a rich multi-wavelength dataset including NuSTAR and VLA, enable precise constraints on various parameters of the emission model, notably the exponential cutoff energy. On the other hand, despite its brief duration, V6598 Sgr exhibited a unique spectral shape compared to other novae observed by the LAT. Interestingly, V6598 Sgr is coincident with a persistent source previously detected by INTEGRAL (IGR J17528-2022) and proposed as an intermediate polar.

I will present a comprehensive analysis of these multi-wavelength observations, as well as the constraints derived on particle acceleration for both novae.

Qi Feng (University of Utah); The VERITAS Collaboration

X-ray and Gamma-ray Follow-up Observations of IceCube Astrophysical Neutrino Alerts

Follow-up observations of IceCube neutrino alerts remain crucial for understanding the origin of high-energy astrophysical neutrinos, despite the challenges in linking single neutrino events to specific astrophysical sources. The hard X-ray band proves particularly valuable, offering strong constraints on the hadronic processes responsible for neutrino production. Gamma-ray observations, on the other hand, shed light on jet activity and offer opportunities to explore potential connections (or their absence) between jets and neutrinos. We present recent results from a multi-wavelength target-of-opportunity program that includes data from the Swift, NICER, NuSTAR, Fermi-LAT, and VERITAS observatories. We focus on the observations of the blazar PKS 0446+11, triggered by the neutrino alert IceCube-240105A in January 2024. The distant blazar exhibited elevated X-ray and gamma-ray fluxes concurrent with the neutrino detection. We will examine the broadband spectral energy distribution of the blazar, interpreted through both leptonic and lepto-hadronic scenarios. In addition, we will present other recent target-of-opportunity observations following up IceCube alerts with NuSTAR, Fermi-LAT, and VERITAS, such as of the blazar PKS 0735+178, located 2.2 degrees away from the best-fit position of the IceCube neutrino event 211208A.

Adi Foord (UMBC)

A High-Energy Perspective on Merging SMBHs

Studying dual and binary active galactic nuclei (AGN) offer a unique window into the complex processes of galaxy mergers and the coalescence of supermassive black holes (SMBHs). Pairs of AGN are expected to be a natural consequence of galaxy-galaxy mergers, and represent crucial stages in galaxy evolution. However, both dual and binary AGN are difficult to detect, and as a result only a handful of confirmed cases exist. In this talk I will review previous studies searching for pairs of AGN across the multi-wavelength spectrum, highlighting how high-energy telescopes offer a unique lens. In particular, X-ray observations have been crucial to identifying possible systems; and in the future, gamma-ray observations can be used to further flag observational signatures that are unique to merging AGN.

Andrea Gokus (Washington University in Saint Louis); Markus Böttcher, Manel Errando, Ivan Agudo, Petra Benke, Florian Eppel, Leonid I. Gurvits, Jonas Heßdörfer, Jeffrey A. Hodgson, Svetlana Jorstad, Matthias Kadler, Yuri Y. Kovalev, Michael Kreter, Mikhail Lisakov, Fe McBride, Jorge Otero Santos, Eduardo Ros, Florian Rösch, Joern Wilms

Uncovering extreme blazar flares at cosmic dawn

High-redshift blazars (z>3) allow us to probe their jets at radio frequencies down to the central black hole due to reduced opacity in the rest frame of the source and to study the accretion processes and black hole growth in the early Universe. However, the detection of gamma-ray emission from these distant sources is difficult - only about a dozen have been detected by Fermi-LAT > 100 MeV. We designed a program utilizing Fermi-LAT data during gamma-ray flares to obtain quasi simultaneous multiwavelength data to study the activity of high-z blazars and ultimately determine the gamma-ray production sites and mechanisms and compare them to the blazars in the local Universe.

In our presentation we will discuss our findings for two high-redshift blazars (z > 4) during extremely luminous flares and include our findings from a high-frequency VLBI campaign for one of the sources. Based on gamma-ray luminosity, those flares are among the brightest detected with LAT so far. The multiwavelength data can be well described by a one-zone leptonic model with parameters in agreement with the VLBI data, and which requires black hole masses > 10^9 solar masses based on the signature from the accretion disk. In addition, we find that the quantifiable variability is strongest in the infrared, and we measure a significant fraction of polarization in the optical R band from the underlying synchrotron emission component.

Andrea Gokus (Washington University in Saint Louis); Knud Jahnke, Paul M. Woods, Vanessa A. Moss, Volker Ossenkopf-Okada, Elena Sacchi, Adam R. H. Stevens, Leonard Burtscher, Cenk Kayhan, Hannah Dalgleish, Victoria Grinberg, Travis A. Rector, Jan Rybizki, Jacob White

A look into the sustainability and inclusiveness of astronomy meetings

In 2019, astronomers traveled a total distance of more than 1.5 Astronomical Units to attend in-person meetings all over the globe, resulting in an average of 1.0 +/- 0.6 tons of CO2-equivalent emissions per participant per meeting. While meetings including personal contact and direct networking opportunities are perceived as more effective in fostering collaboration, they are significantly less sustainable and inclusive compared to virtual formats, which, with the right tools and setup, can be as successful. We will present our assessment of the greenhouse gas emissions related to all Astronomy meetings in 2019, and discuss how we can use our results to establish a better path forward to make Astronomy more inclusive and sustainable for future meetings.

Adam Goldstein (USRA); W. Cleveland, S. Bala, C. Fletcher, O. Roberts, C. M. Hui, D. Kocevski, C. Wilson-Hodge, J. Wood

The Gamma-ray Data Tools: An open-source generalized analysis toolkit for space-based gamma-ray instruments

Over the last several years, the increasing community interest in Fermi GBM data led to the development of the Fermi GBM Data Tools, a toolkit that enables community analysis of all public GBM data. Through various grants, and now supported by NASA HQ funding, we have generalized the GBM Data Tools into the Gamma-ray Data Tools (GDT; https://astro-gdt.readthedocs.io/en/latest/) that are able to support other missions that produce data similar to GBM. The GDT currently supports data from instruments on seven legacy and current missions in addition to GBM: CGRO/BATSE, HETE-2/FREGATE, INTEGRAL/SPI-ACS, MAXI/GSC, RXTE/ASM, and Swift/BAT. We are in the process of adding Suzaku/WAM, BurstCube, Glowbug, and the upcoming StarBurst Multimessenger Pioneer to the GDT family. Additionally, the GDT serves as an important backbone for the open-source InterPlanetary Network and Multi-mission Targeted Search packages under development at NASA Marshall Space Flight Center and partners. I will provide an overview of the current capabilities of the GDT, review the current state and roadmap of the GDT, and outline the role the community has in guiding the evolution of the GDT to serving community interests in time-domain and multi-messenger astronomy.

Rahul Gupta (NASA/GSFC); Prof. A. J. Castro-Tirado, Prof. S. B. Pandey, Prof. D. Bhattacharya, Prof. J. Racusin, Prof. V. Bhalerao, Dr. T. Chattopadhyay, Ms S. Gupta, Dr. V. Sharma, Dr. S. Iyyani, Dr Santosh. V. Vadawale; AstroSat CZTI

A detailed time-resolved and energy-resolved spectro-polarimetric study of bright GRBs observed using Fermi and AstroSat

The radiation mechanism of the prompt emission is still an open issue and can be resolved using a systematic and uniform time-resolved spectro-polarimetric study. In this work, we performed a comprehensive investigation of the spectral, temporal, and polarimetric characteristics of five bright GRBs observed using the AstroSat CZTI, Fermi GBM and Swift BAT to provide insight into the prompt emission radiation mechanisms. These GRBs are detected by CZTI during its first year of operation, and the average polarisation characteristics of these GRBs are already published in Chattopadhyay et al. 2022. In the present work, we investigated the time-resolved (in 100-600 keV) and energy-resolved polarization measurements of these GRBs with an improved polarimetric technique such as increasing the effective area and bandwidth (by using data from low-gain pixels), using an improved event selection logic to reduce noise in the double events and extend the spectral bandwidth. In addition, we also carried out detailed time-resolved spectral analyses of these GRBs using empirical and physical synchrotron models. By these improved time-resolved and energy-resolved spectro-polarimetric studies, we could pin down the elusive prompt emission mechanism of these GRBs. Our spectro-polarimetric analysis reveals that GRB 160623A, GRB 160703A, and GRB 160821A have a Poynting flux-dominated jet. On the other hand, GRB 160325A and GRB 160802A have a baryonic-dominated jet with mild magnetization. Furthermore, we observe a rapid change in polarization angle by ~ 90 degrees within the main pulse of very bright GRB 160821A, consistent with our previous results. Our study suggests that the jet composition of GRBs may exhibit a wide range of magnetization, which can be revealed by utilizing spectro-polarimetric investigations of the bright GRBs.

Soumya Gupta (Bhabha Atomic Research Center, Mumbai, India / Homi Bhabha National Institute, Mumbai, India); Rahul Gupta, Tanmoy Chattopadhyay, Dipankar Bhattacharya, A R Rao, Sunder Sahayanathan, Varun Bhalerao and Santosh Vadawale; Astrosat CZTI

Exploring the radiation mechanism of extremely energetic GRB 230307A using Specto-polarimetry observations

GRB 230307A is the second-most brightest long burst found associated with Kilonova emission, providing a unique opportunity to explore the open question on radiation mechanism and jet composition of Gamma-ray bursts (GRBs). In this work, we examine the temporal, spectral and polarimetric characteristics of the high-energy radiation emitted by GRB 230307A based on prompt emission spectro-polarimetric analysis using joint AstroSat Cadmium Zinc Telluride Imager (CZTI), Fermi Gamma-ray Burst Monitor (GBM) and Konus-Wind observations. Our time-resolve spectro-polarimetric analysis of the burst revealed the transition from Baryonic to Poynting flux-dominated jet composition within the burst's duration. Additionally, we compared the high-energy properties of GRB 230307A with GRB 211211A and noted that both of these nearby long-Kilonovae-connected bursts have similar temporal and spectral prompt behaviour.

John Hewitt (University of North Florida); S. Kumar, B. Humensky; Fermi LAT collaboration

Radio-dim, gamma-ray-bright supernova remnants

We report the Fermi-LAT detection of two newly established supernova remnants (SNRs) which belong to a class of radio-faint, yet gamma-ray bright SNRs. SNR G189.6+3.3 overlaps with the smaller and brighter SNR IC 443. The X-ray emission shows a radiative recombining continuum, evidence of an ongoing or past interaction with a dense medium. The gamma-ray emission is extended, matching the size of the SNR in radio and X-rays, with a spectral index of 2. The SNR candidate G107.5-5.2, also named the Nereides Nebula, is detected in [O III] and Hα narrow-band images. Its large 3 degree diameter in optical makes it comparable to the Cygnus Loop, albeit much fainter. It is spatially coincident with the extended gamma-ray source FHES J2304.0+5406. No radio or X-ray emission has yet been detected. Their large gamma-ray extensions are well-matched to the sizes of the SNRs, supporting an association.

Sam Hori (University of Wisconsin-Madison); Justin Vandenbroucke, Abhishek Desai; IceCube Collaboration

Stacking Search for IceCube Neutrinos from Fermi-LAT Active Galactic Nuclei

Active galactic nuclei (AGN) are a promising potential class of cosmic ray accelerators. AGN are the most common source class detected by Fermi-LAT, and associating neutrinos with them can prove that they are hadronic accelerators. Understanding the connection between gamma ray and neutrino emission would elucidate the physics in AGN, including the acceleration processes of high energy cosmic rays and the environment in which they are produced. The IceCube Neutrino Observatory is sensitive to TeV-PeV neutrinos associated with hadronic processes in cosmic-ray accelerators. IceCube has detected a diffuse astrophysical neutrino flux, but the origins of this flux remain mostly unclear. The first two detected sources of IceCube neutrinos (blazar TXS 0506+056 and type 2 Seyfert galaxy NGC 1068) are both detected in gamma rays by Fermi-LAT. Previous studies have set upper limits on the AGN contribution to the diffuse neutrino flux under various model assumptions. There appear to be multiple populations of neutrino sources, each contributing a fraction of the total diffuse flux. I present an analysis using an updated Fermi-LAT catalog and larger IceCube data sample and improved statistical methods (especially improvements to the point spread function not used in previous AGN population studies) to perform a stacking search for neutrinos originating from Fermi-LAT AGN.

Kun Hu (Washington University in St. Louis); XL-Calibur collaboration; XL-Calibur collaboration

First Results from the 2024 Flight of the XL-Calibur Hard X-ray Polarimetry Mission

XL-Calibur is a balloon-born telescope that measures the hard X-ray polarization of the 15 - 80 keV emission from bright X-ray sources like accreting black holes and neutron stars. The telescope uses a 12-meter-focal-length X-ray mirror to collect photons into a scattering CZT polarimeter covered by an active shield. The Wallops Arc Second Pointer (WASP) pointing system provides arc-second precision in posting the telescope to the sources. XL-Calibur is scheduled to launch in Summer 2024 from Esrange in Sweden for a long-duration balloon flight from Sweden to Canada. The experiment will observe several bright X-ray sources including Cyg X-1, Cyg X-3, and the Crab Nebula, with contemporary observations from IXPE, NICER, and NuSTAR. I will give a general review of the XL-Calibur mission, and discuss its performance during the 2024 balloon campaign. If available at the time of the conference, I will present the first results from the observations. The XL-Calibur polarization results will be of great interest to future gamma-ray polarization missions such as COSI.

Michelle Hui (NASA MSFC); FIG SAG

Future of gamma-ray science in space

Gamma rays are produced by some of the most extreme processes in the Universe. Their observations provide critical insights into a broad range of astrophysical phenomena and markers of fundamental physics. Gamma-ray astronomy probes the acceleration of charged particles, the decay of radioactive isotopes or subatomic particles, and potential signatures of new physics. These are closely linked to the origins of gravitational waves, neutrinos, and cosmic rays. In turn, gamma-ray observations play a pivotal role in constructing a comprehensive multimessenger image of both transient and steady high-energy emission in the Universe.

The space-based gamma-ray regime spans from tens of keV to hundreds of GeV, bounded by the X-ray and ground-based gamma-ray regimes. The gamma-ray astronomy community has benefitted from a continuous set of capable missions leading to a number of remarkable discoveries that have revolutionized astrophysics. The combined wealth of recent discoveries and the need to identify next-generation missions places gamma-ray astronomy in an ideal position to reassess its future priorities. The Future Innovations in Gamma-ray Science Analysis Group (FIG SAG) was initiated this year to gather community inputs to identify future science drivers, necessary capabilities, and priorities for the future of gamma-ray astronomy. In this talk, I will present our current findings based on the community's vision of gamma-ray astronomy looking towards 2040.

Brian Humensky (NASA Goddard & Univesity of Maryland - College Park); VERITAS and Fermi-LAT

Resolving the gamma-ray SNR IC 443 with Fermi LAT and VERITAS

IC 443 is among the closest and best-studied cases of a supernova remnant interacting with a molecular cloud. The gamma-ray spectrum shows a "pion bump" cutoff at low energies characteristic of a hadronic origin. New gamma-ray observations with Fermi-LAT and VERITAS resolve the SNR into an extended shell, matching the general morphology seen at other wavelengths. This allows the first spatially resolved study of gamma-ray emission from GeV to TeV energies. A spectral break is evident from GeV to TeV energies for both the entire remnant and individual regions. The morphology is largely uniform from GeV to TeV, with some differences uncovered for the brightest region above 1 TeV.

Nazma Islam (NASA GSFC and UMBC)

Multi-wavelength observations of the Candidate Redback 4FGL 1702.7-5655

Millisecond pulsars, characterized by short pulse and orbital periods, are believed to have descended from Low Mass X-ray binaries. Some of these binary pulsars emit gamma-rays and have been detected by Fermi-LAT. These gamma-rays likely originate from the Intra-Binary shock (IBS) between the pulsar and its companion's winds or during the transition between accretion and rotationally-powered states. We present results from a multi-wavelength campaign using the Fermi-LAT, X-rays and radio observations of the candidate redback 4FGL 1702.7-5655. Through Chandra observations, we confirmed the X-ray counterpart and analyze its X-ray spectrum. Long-term Fermi-LAT observations revealed significant changes in the orbital modulation of the source, particularly in the narrow eclipses, likely related to variations in the IBS within the system

Scott Joffre (Clemson University); Dr. Lea Marcotulli, Dr. Chris Karwin, Dr. Marco Ajello; Fermi-LAT Collaboration

The Second Fermi-LAT low-energy catalog (2FLE)

The Fermi Large Area Telescope (Fermi-LAT) has been continuously observing the sky from 20 MeV to 1 TeV for more than 15 years. Although Fermi-LAT's sensitivity reaches down to 20 MeV, its low-energy range has been largely left under-explored. As we await an all-sky MeV mission such as COSI, it is now the prime time to capitalize on the full capabilities of Fermi-LAT. To complement and improve on the first Fermi-LAT low-energy catalog (1FLE), we have developed a specialized analysis using 14 years of the LAT data with the best angular reconstruction (Pass8 PSF3) to construct a sensitive catalog of point sources between 20-200 MeV. This program will start bridging the gap between the MeV and GeV energy bands, strongly enhancing the legacy of the Fermi mission and supporting the scientific case for future all-sky MeV missions.

Svetlana Jorstad (Institute for Astrophysical Research, Boston University, Boston, MA, USA); Alan P. Marscher, Jose L. Gomez, Mark A. Gurwell, and Sergey S. Savchenko

Dramatic Multi-Wavelength Activity of BL Lacertae in 2020-2023

We present an analysis of multi-wavelength behavior of the blazar BL Lacertae during its unprecedented activity in 2020-2023. The analysis includes a gamma-ray light curve obtained with the Fermi Large Area Telescope (LAT) at 0.1-200 GeV, an X-ray light curve at 0.3-10 keV constructed using Neil Gehrels Swift X-ray Telescope (XRT) data, optical photometric and polarimetric observations in R band, Sub-Millimeter Array (SMA) flux measurements at 1.3 mm, and monthly total and polarized intensity images obtained with the Very Long Baseline Array (VLBA) at 7 mm. The gamma-ray light curve reveals at least 7 bright outbursts with a duration of several weeks each. All of these have optical counterparts, while the X-ray light curve contains only 4 flares coinciding with the 4 brightest gamma-ray outbursts, perhaps due to sparser X-ray data. The gamma-ray and optical light curves correlate without a measured delay within 6 hrs, with a correlation coefficient of 0.6, significant at the $5\sigma$ level. There is no significant correlation between the gamma-ray light curve and that at either 1 mm or 7mm (obtained by integration over the total intensity images). The radio light curves correlate extremely well, with a correlation coefficient of 0.87. Both radio light curves show a gradual increase of flux beginning in 2020 January, with culmination in the middle of November 2023, when the mm-wave flux density exceeded 20 Jy. We will discuss also the polarization behavior in R band and at 7~mm, as well as activity in the jet. The multi-wavelength behavior supports the idea proposed by Raiteri et al. (2023) that different mechanisms are responsible for the variability in BL Lacertae on long (months) and short (day, hours) time scales, with orientation effects playing a major role on long time scales and shocks/reconnections/instabilities/turbulence determining the short timescale behavior. This research is funded in part by a number of NASA Fermi Guest Investigator grants, the latest of which are 80NSSC23K1508 and 80NSSC23K1507.

Constantinos Kalapotharakos (NASA, Goddard Space Flight Center); G. Olmschenk, T. Lechien, E. Broadbent, Z. Wadiasingh, D. Skiathas, D. Kazanas, A. Harding

Revolutionizing the inference of mass, radius, and magnetic field structure of neutron stars using NICER and Fermi-LAT data

We present neural networks (NNs) that accurately reproduce model thermal X-ray light curves and non-thermal gamma-ray light curves corresponding to force-free multipolar magnetic field structures. By integrating these NNs into our Markov chain Monte Carlo codes, we achieve a remarkable acceleration (over 300 times faster) in inferring the mass, radius, and magnetic field structure of millisecond pulsars from their NICER and Fermi-LAT light curves. Our study highlights the crucial role of Fermi-LAT light curves in resolving degeneracies, enabling more precise determinations of pulsar characteristics. We will also discuss the next steps and challenges in this field, as well as the broader implications of our findings for pulsar research and astrophysics. This innovative approach not only streamlines the analysis process but also opens new avenues for understanding the complex magnetic field structures of millisecond pulsars and the fundamental properties of dense matter.

Bidzina Kapanadze (E. Kharadze National Astrophysical Observatory (Abastumani); Space Research Center, Department of Astronomy and Astrophysics, School of Natural Sciences and Medicine, Ilia State University (Tbilisi, Republic of Georgia))

Long-Term Fermi Observations of Markarian 421: Physical Clues

Markarian 421 is the brightest LAT-band source among the High-energy-peaked BL Lacertae objets (HBLs) and detectable even on intraday timescales during the strongest high-energy flares. The 0.3-300 GeV photon flux frequently was higher than 10^{-7} ph/cm^2/s, which has been observed very rarely for other HBLs. The strongest long-term flaring activity was recorded during 2012 March -2013 October and 2017 June - 2018 July, plausibly related to the enhanced matter collimation rate through the jet pointed to the observer. On shorter timescales, the source showed gamma-ray flares characterized by symmetric or two-peak profiles, as well by positive and negative asymmetries. Symmetric shape of the flare indicates that the observed variability was driven by the crossing timescale of the underlying disturbance (e.g., relativistic shock front). A two-peak flare can be established by the propagation of forward and reverse shocks triggered by colliding "shells" of high-energy plasma, injected into the blazar jet with different speeds. Flares with a negative asymmetry may indicate a gradual acceleration of the particles, which are responsible for the IC upscattering of low-energy photons to the MeV-to-GeV range. Nonuniformity of the Doppler factor across the jet (caused by the radial expansion of the relativistic flow) may produce an asymmetric flare profile with substantially extended decay phase positive asymmetry. The 0.3-300 GeV spectra sometimes were extremely hard that is expected if there was a significant contribution of the gamma-ray photon from the hadronic cascades.

Chris Karwin (NASA GSFC); Alex McDaniel, Marco Ajello, Mattia Di Mauro, Alex Drlica-Wagner, Miguel A. Sanchez-Conde; Fermi-LAT

Legacy Analysis of Dark Matter Annihilation from the Milky Way Dwarf Spheroidal Galaxies with 14 Years of Fermi-LAT Data

The Milky Way dwarf spheroidal satellite galaxies (dSphs) are particularly powerful targets to search for gamma rays from dark matter (DM) annihilation or decay. They are nearby, DM-dominated, and lack significant emission from standard astrophysical processes. Previous studies using the Fermi Large Area Telescope (LAT) of DM-induced emission from dSphs provide some of the most stringent constraints on DM properties, such as the annihilation cross section and mass. We report here on an analysis of the Milky Way dSphs using over 14 years of LAT data along with an updated census of dSphs. While no significant signal is detected, the combined likelihood analysis of the dSphs shows the presence of a 2-3 local sigma signal at a DM mass of 150-230 GeV. We project that this signal, if real, could reach the 4-sigma confidence level with an additional 10 years of observations.

Alexander (Sasha) Kashlinsky (NASA's GSFC and UMd)

Probing the dipole of the gamma-ray background

We measured the dipole of the diffuse γ-ray background (DGB), identifying a highly significant time- independent signal at 3-100 GeV coincidental with that of the Pierre Auger UHECR. The DGB dipole is determined from flux maps in narrow energy bands constructed from 13 yr of observations by the Large Area Telescope (LAT) of the Fermi satellite. The γ-ray maps were clipped iteratively of sources and foregrounds similar to that done for the cosmic infrared background. The γ-ray sky's dipole/monopole ratio is much greater than that expected from the DGB clustering component and the Compton-Getting effect origin with reasonable velocities. At ~(6.5-7)% it is similar to the Pierre Auger UHECRs at >8 EeV, pointing to a common origin of the two dipoles. However, the DGB flux associated with the found DGB dipole reaches parity with that of the UHECR around EUHECR <1 EeV, perhaps arguing for a non-cascading mechanism if the DGB dipole were to come from the higher-energy UHECRs. The signal-to-noise ratio of the DGB dipole is largest in the 5-30 GeV range, possibly suggesting the γ-photons at these energies are the ones related to cosmic rays.

Matthew Kerr (US Naval Research Laboratory); Fermi LAT Collaboration

Do Gamma-ray Pulsars Mode Switch?

Over the past 15 years, it has become clear that many radio pulsars switch between long lived (minutes to years) magnetospheric states with different spindown rates and pulse profiles. The nature of these states and the mechanism regulating the switching remain unknown, but determining the origin would provide insight into the operation of pulsar magnetospheres. Gamma-ray pulsars, on the other hand, are almost completely stable: of more than 300 known pulsars, only PSR J2021+4026 exhibits similar behavior. Why one and only one? It may be that other cases are hiding in the LAT pulsar sample, because detecting state switching on short timescales requires new methods. Here, we present results from a comprehensive search of over 100 LAT-detected pulsars for flux changes on all timescales down to a few minutes. We further extend this search with a study of gamma-ray pulse profile stability.

Nikita Khatiya (Clemson University); Karwin, C., Boughelilba, M., Ajello, M., Reimer, A. and Hartmann, D.

Characterizing the γ-ray Emission from Low Luminosity Active Galactic Nuclei

A majority of the active galactic nuclei (AGN) population in the local Universe comprises low luminosity AGNs (LLAGNs), accreting at very low Eddington rates. Radio observations at high angular resolution show evidence of extended radio emission in the form of relativistic jets. Studies have predicted that both the jets and disks of LLAGN are capable of generating gamma rays. However, only 4 significant gamma-ray sources have been detected to date. To gain insight into this source population, we conduct a stacking analysis of the subthreshold sample from the Palomar survey and broadband SED modeling for all sources (both subthreshold and significant). Our preliminary results indicate a brand new source detection apart from the previously detected sources. We also detect a statistically significant gamma-ray signal from the subthreshold sample. Additionally, we test for a radio-gamma luminosity correlation, extending it to ~100x lower radio luminosities compared to FR0, FRI, and FRII radio galaxies. In this talk I will discuss these results, as well as the implications for the gamma-ray emission mechanisms.

Bruno Khélifi (APC, CNRS/Université Paris Cité) Gammapy

Open analysis librairies for the high-energy astrophysics

With the advent of open data in the field of high-energy astronomy, the community is organizing itself to offer to users open libraries to process different types of data and to simulate data. This presentation will review the efforts in open research software for the gamma-ray observatories, from instrument or astrophysical simulation tools to the observatory data processing libraries. With their technical features, the projects' organization will be synthesized. Their diversity will be highlighted with the various and evolving data and software management plans of observatories and with the institutional Open Science policies.

Daewon Kim (Max-Planck-Institute for Radio astronomy); AGN / LAT Collaboration

The Role of VLBI in LAT studies of Gamma-ray bright AGN

One of the primary goals of the Fermi Large Area Telescope (LAT) is to broaden our understanding of the Gamma-ray emission processes in astronomical objects. Blazars are a major Gamma-ray source in the extragalactic sky. Gamma-ray emission from blazars is strong and highly variable with time. It is believed that the relativistic jets emanating from supermassive black holes in blazars produce the Gamma-ray emission within parsec-scale distances from the central engine. Unfortunately, this scale requires a high level of angular resolution below milliarcseconds (mas) and that is far beyond a resolving power of the LAT. To spatially resolve the sources and look directly into the high-energy production sites in the jets, we need observations with Very Long Baseline Interferometry (VLBI). The VLBI+LAT studies aim to explore the Gamma-ray origin in blazar jets by finding a connection between VLBI-scale jet activity and Gamma-ray emission. In this presentation, I introduce some of the science cases with my recent/ongoing VLBI+LAT studies of several Gamma-ray bright blazars. Source activities at Gamma-rays were monitored by the LAT and high-resolution VLBI images of the jets at submas scales were used to see what happened in the inner jet regions during a flaring state at LAT Gamma-ray bands. I discuss what the observational results could imply on the nature of the Gamma-ray origin in the jets.

Daniel Kocevski (NASA)

The StarBurst Multimessenger Pioneer

The StarBurst Multimessenger Pioneer is a highly sensitive wide-field gamma-ray monitor designed to detect the prompt emission of short gamma-ray bursts (SGRBs), a key electromagnetic (EM) signature of neutron star (NS) mergers. StarBurst is designed to capitalize on the new era of multimessenger astronomy by utilizing the advancements in gamma-ray detectors made over the last decade. With over 400% the effective area of the Fermi GBM and full coverage of the unocculted sky (8 Sr), StarBurst will make highly sensitive observations of EM counterparts to NS mergers and be a key partner to the GW network in discovering NS mergers. StarBurst is designed as a SmallSat to be deployed to Low Earth orbit as a secondary payload using the ESPA Grande interface for a nominal 1-year mission starting in 2027. StarBurst relies heavily on the heritage of the GBM and Glowbug instruments, consisting of an array of 12 CsI(TI) scintillator detectors that utilize new, low mass and low voltage, Silicon photomultipliers (SiPMs) to cover an energy range from 30 keV to 2 MeV. This presentation will give an overview of the mission, its current development status, and possible synergies between StarBurst and Fermi-GBM and Fermi-LAT during the LIGO/Virgo O5 run.

Sajan Kumar (University of Maryland, College Park); Xiaojie Wang, Michael Martin; "VERITAS, HAWC, and XMM-Newton

Investigating the origin of gamma-ray emission from the unidentified PeVatron LHAASO J2108+5157 using data from VERITAS, HAWC, Fermi-LAT and XMM-Newton

Understanding the nature of Galactic PeVatrons, which accelerate cosmic rays to energies up to 10^15 eV, remains a challenging problem in very high-energy gamma-ray astronomy. Although supernova remnants (SNRs) are considered prime candidates for Galactic PeVatrons, definitively associating SNRs with PeVatrons requires further exploration. The LHAASO collaboration has recently reported the detection of several ultra-high-energy gamma-ray sources emitting photons above 100 TeV. One of these sources, LHAASO J2108+5157, is under investigation due to the lack of a clear counterpart in other wavelengths. Our presentation will include observations from VERITAS, HAWC, Fermi-LAT, and XMM-Newton, as well as multiwavelength modeling, aimed at understanding the origin of gamma rays from LHAASO J2108+5157.

Anu Kundu (Centre for Space Research, North-West University); Constantinos Kalapotharakos, Alice K. Harding, Demosthenes Kazanas, Christo Venter, Zorawar Wadiasingh

The retarded multipolar magnetic field of millisecond pulsar J0030+0451

Recent observations of millisecond pulsars by the Neutron star (NS) Interior Composition ExploreR (NICER) telescope have led to precise estimates of their mass and radius. The analysis strongly suggested consideration of a multipolar magnetic field for PSR J0030+0451 (J0030), and it also localized its surface hot spots. Kalapotharakos et al. (2021) used a vacuum offset static dipole plus quadrupole magnetic field description to fit the NICER X-ray light curves (LCs) of J0030, and subsequently, a force-free field configuration to also fit the gamma-ray LCs seen by the Fermi-LAT.

We have developed a code to implement a generic multipolar magnetic field expansion to constrain the magnetic field parameter space for J0030, adopting retarded vacuum field solutions (Pétri 2015), as opposed to previous works based on static fields, since these are closer to the more realistic force-free configuration required to simultaneously explain and fit the NICER X-ray and Fermi gamma-ray LCs. We consider all sub-components up to the octupole component of this centered field, and employ Markov chain Monte Carlo (MCMC) methods to fit the LCs of J0030. We constrain the number of field components and orientation required for an adequate fitting.

We aim to eventually expand our parameter space by keeping the stellar mass and radius free, allowing us to place more robust constraints on the equation of state. We present the description of the multipolar magnetic field configuration, and show the MCMC solutions for the optimal field required to fit the NICER X-ray and Fermi gamma-ray LCs of J0030.

Naoko Kurahashi Neilson (Drexel University); "IceCube Collabroation, P-ONE collaboration, KM3NeT collaboration

Neutrino Astronomy and Fermi: The Past, The Present, and The Future

In the past decade, neutrino astronomy went from dream to reality with IceCube producing spectacular observations of the very first neutrino sources in the sky. Last year, the Galactic Plane was observed in neutrinos, making it the first non-electromagnetic view of our own galaxy. In all these discoveries, Fermi observations were key. Fermi has been the observatory almost entirely responsible for tethering the field of neutrino astronomy to astronomy. In this talk, I will review the successes of neutrino astronomy in the past decade, and how Fermi played a role in all of them. The current state of neutrino astronomy is evolving, and future neutrino telescopes that are planned all assume a Fermi or Fermi-like counterpart to be operational. This talk will make a case for the ideal and necessary future gamma-ray observatory to sustain the future of neutrino astronomy.

Alex Lange (George Washington University); Jordan Eagle, Oleg Kargaltsev

The Vela pulsar and its pulsar wind nebula Vela-X using 13 years of Fermi-LAT Observations

We present findings from over 13 years of data analysis using the Fermi-LAT telescope on the Vela pulsar, covering energies from 60 MeV to 10 GeV, and its pulsar wind nebula (PWN), Vela-X, for energies above 1 GeV during off-pulse phases. The Vela-X PWN is best described with two components: a large radial Gaussian and a smaller, offset radial disk, both having a similar spectral index of about 2.3. This suggests they likely share a common PWN origin, though a supernova remnant (SNR) component is also possible for the compact disk. Using an updated Vela-X model, we examined the spectral properties and their evolution of the Vela pulsar through phase-resolved data analysis. We looked at features like the peak energy, the width at the peak energy, and the spectral index at 100 MeV. The best-fit spectral models from each pulse peak (Peak 1 and Peak 2) were extended to UV energies and compared with archival data for UV, X-ray, soft gamma-ray, and TeV energies. We discuss the physical implications of these findings based on the model and data comparisons.

Adrien Laviron (Laboratoire Leprince-Ringuet, IPP, CNRS/IN2P3); Denis Bernard, Philippe Bruel; Fermi-LAT

Polarimetry of the Vela pulsar with the Fermi-LAT: First results

The polarization of high-energy photons carries a wealth of information about the structure of gamma-ray sources, but so far only limited measurements in the MeV band have been made. The Fermi Large Area Telescope (Fermi-LAT) detects gamma rays between ~30,MeV and ~300,GeV, using their conversion into an electron-positron pair, with the linear polarization affecting the azimuthal direction of the pair. Measuring this azimuthal direction is extremely challenging due to the multiple scattering of the electron and positron in the detector. We developed a dedicated event reconstruction to perform this measurement. Simulations show that we can expect ~ 20, uncertainty on the polarization fraction of the Vela pulsar. In this talk, I will present the first results of this method, obtained with 15 years of Vela pulsar data.

Ioannis Liodakis (NASA Marshall Space Flight Center); "Fermi, IXPE

AGN in the discovery era of X-ray polarimetry

Supermassive black holes form the most intriguing astrophysical systems offering countless opportunities to study fundamental physics in regimes not accessible to laboratories on Earth. Their multimessenger emission manifests in the formation of accretion disks, jets, and the acceleration of extremely energetic particles all of which are still poorly understood. X-ray polarization is a crucial probe of the geometry, magnetic field structure, and emission processes in active galactic nuclei (AGN). Until now, polarization observations have been limited to the radio and optical regime. The recently launched Imaging X-ray Polarimetry Explorer -- IXPE, the first X-ray polarization mission to target AGN, offers radically new avenues of studying high-energy processes in the Universe. I will discuss the polarization observations and results from the first two years of IXPE observations trying to understand active supermassive black holes.

Francesco Loparco (INFN Bari and Bari University); M. N. Mazziotta; Fermi-LAT collaboration

The gamma-ray Moon seen by the Fermi LAT

The Moon is the brightest source of gamma rays in the Solar System. Gamma rays from the Moon are produced in the hadronic interactions of cosmic-ray nuclei with the lunar surface, and their flux is sensitive to solar activity, which modulates the charged cosmic-ray fluxes in the Solar System. We have measured the gamma-ray flux from the Moon using the data collected by the Fermi Large Area Telescope in its first 15.5 years of operation, which cover a period exceeding the duration of a whole solar cycle. We have performed an overall measurement of the average spectrum as well as individual measurements in 6-month time intervals, finding a strong correlation with the solar activity. We have also implemented a simulation of the interactions of cosmic-ray protons and helium nuclei with the Moon based on the FLUKA code. We find that the gamma-ray fluxes measured by the LAT are well reproduced by those evaluated by our simulation assuming proton and helium spectra at Earth as measured by PAMELA and AMS-02.

Francesco Loparco (INFN Bari and Bari University); M. Giiberti, M. Nicola Mazziotta; Fermi-LAT

Search for line-like and box-like features in the Galactic gamma-ray spectra with the Fermi LAT

Indirect dark matter searches with gamma rays are performed looking for features in the spectra that could arise from the annihilation or decay of dark matter particles in space. In this work we present the results of a search for line-like and box-like features in the gamma-ray spectra in five sky regions centered on the Galactic Center, optimized for different DM density profiles and annihilation or decay channels, performed using a 15-year dataset collected by the Fermi Large Area Telescope in the energy range from 1 GeV to 1 TeV. Line-like features could arise from the direct annihilation of pairs of dark matter particles into pairs of gamma rays. Box-like features could be due to the annihilation of pairs of dark matter particles into pairs of low-mass mediators, which in turn decay into pairs of gamma rays. In both scenarios, the intensity of the feature is related to the velocity-averaged dark matter annihilation cross section. Although we have not found any evidence of such features, we have been able to set constraints on their intensities, which have been converted into constraints on the velocity-averaged cross sections.

Benoit Lott (LP2IB/IN2P3); Fermi-LAT collaboration

Low-latitude unassociated Fermi-LAT sources: a long-standing puzzle

The unassociated population represents about 30% of the sources in the latest 4FGL-DR4 release, with about one half lying at low (|b|<10°) Galactic latitudes. Many of these low-latitude sources exhibit properties that set them apart from established classes of Galactic gamma-ray emitters, in particular very soft spectra. The latter feature earned them the denomination "soft Galactic unassociated sources" (SGUs). New classes may hide in this population but the possibility that a sizable fraction of the SGUs come from mismodeled diffuse emission cannot currently be ruled out. The presentation will give an overview of this issue.

Silvia Manconi (LAPTh, CNRS France)

The Galactic center excess at the highest energies: morphology and photon-count statistics

Since more than 10 years, an unexpected gamma-ray component over astrophysical backgrounds has been detected at GeV energies towards the Galactic center in the data of Fermi-LAT. Initially, this excess was considered to be hinting at GeV thermal relics annihilating in the Galactic dark matter halo. However, recent works have demonstrated that the excess is better explained by a population of millisecond pulsar-like sources in the Galactic bulge. While the excess photon flux is peaked at about few GeV, a significant high energy tail extending up to tens of GeV has been detected. Such high-energy photons are naturally explained by the inverse Compton emission of electrons and positrons emitted by millisecond pulsars. I will present a novel study of the morphology and photon statistics of the Galactic center excess at energies above 10 GeV. This is based on an innovative method, which combines adaptive template fitting and pixel-count statistics in order to assess the role of sub-threshold point sources while minimizing the mis-modelling of Galactic diffuse emission backgrounds. Our results bring further support to the attempt of explaining, at least partially, the high-energy tail of the excess in terms of a population of point sources, likely corresponding to millisecond pulsars.

Peter Marinos (KIPAC); T.A. Porter, G.P. Rowell, G. Jóhannesson, I.V. Moskalenko

Variability of the Galactic CRs and Diffuse Gamma-Ray Emission Predicted with GALPROP

Using the 3D simulation software GALPROP we modelled Galactic cosmic-ray (CR) diffusion utilising a distribution of stochastically placed CR sources. This source distribution more accurately represents the formation rates and finite lifetimes compared to the steady-state CR injection models that are typically assumed. We investigate the time variability of the diffuse gamma-ray emission along the Galactic plane.
Our results show that the leptonic component of the gamma-ray emission is highly sensitive to the assumed electron injection and spectral characteristics. Furthermore, the leptonic component is heavily dependent on the positions of the sources due to the rapid synchrotron cooling of the very-high-energy electrons. As the gamma-ray energy increases so does the magnitude of the variations in the total diffuse flux. At 100 GeV the variations due only to the stochastic nature of the CR source placement can be as large as 30 percent in the outer Galaxy, increasing to an order of magnitude at 10 TeV.
Considering the placement of the CR sources is critical in connecting the Galactic diffuse emission over nine decades of energy -- from the GeV emission observed by Fermi-LAT, to the TeV emission observed with H.E.S.S., and to the PeV emission observed by LHAASO. The variations induced by the stochastic CR source placement are in agreement with the H.E.S.S. Galactic plane survey. Additionally, the temporal variations provide a natural explanation for the observed excess gamma-ray emission measured by LHAASO without ad-hoc tuning of the models.

Israel Martinez-Castellanos (UMD/NASA-GSFC/CRESST); COSI

The Compton Spectrometer and Imager (COSI): opportunities for joint analyses with Fermi

The Compton Spectrometer and Imager (COSI) is a selected Small Explorer (SMEX) mission launching in 2027. It consists of a wide field-of-view Compton telescope that will probe with increased sensitivity the under-explored MeV gamma-ray sky (0.2-5 MeV), aiming to uncover the origin of the Galactic positrons, mapping the sites of Galactic nucleosynthesis, improving our understanding of accreting black holes through polarization measurements, and detecting and localizing gravitational wave counterparts. COSI will be greatly complementary to Fermi thanks to its polarization measurement capabilities and energy range —slightly below LAT and overlapping with GBM. In this contribution, we will provide an overview of COSI, its science goals, and opportunities for future joint analysis with Fermi. We will also present COSI's publicly available data challenges and the cosipy analysis software, which can be used to perform joint coherent fits with Fermi data through the Multi-Mission Maximum Likelihood framework (3ML).

Zachary Metzler (University of Maryland/NASA GSFC); Lucas D. Smith, Nicholas Cannady, Regina Caputo, A. Wilder Crosier, Carolyn Kierans, Nicholas Kirschner, Emily Kong, Lucas Parker, Adam J. Schoenwald, Daniel Shy, Janeth Valverde, Richard Woolf, Aleksey Bolotnikov, Thomas J. Caligiure, Gabriella A. Carini, Alfred Dellapenna, Jack Fried, Priyarshini Ghosh, Sean Griffin, J. Eric Grove , Elizabeth Hays, Sven C. Herrmann, Iker Liceaga-Indart, Julie McEnery , John Mitchell , A. A. Moiseev, Jeremy Perkins, Bernard Phlips, Makoto Sasaki, Clio Sleator, Sambid Wasti, Eric Wulf, Anna Zajczyk; ComPair

The ComPair Balloon Instrument and Flight

ComPair is a prototype instrument that aims to develop key technologies for a next-generation, large-scale gamma-ray telescope. ComPair consists of four subsystems: a 10-layer double-sided silicon strip detector tracker, a cadmium zinc telluride imaging calorimeter, a cesium iodide calorimeter, and a plastic anti-coincidence detector. ComPair was launched as a balloon payload from Ft. Sumner, NM and completed a 6-hour flight on August 27, 2023. We will report on ComPair's design, balloon flight performance, and first results.

Tsunefumi Mizuno (Hiroshima University); Katsuhiro Hayashi; Fermi-LAT Collaboration

Study of local clouds using HI line profile

Diffuse gamma-ray emission is a powerful probe to study Galactic cosmic rays (CRs) and the interstellar medium (ISM), and the Fermi-LAT has provided high-quality data continuously for over 15 years. Studies of Galactic CRs and the ISM using gamma-ray emission is complicated by the uncertainty of the interstellar gas density, and using HI-line width is a possible way to overcome this difficulty. We obtained CR/ISM properties of the molecular clouds MBM 53, 54, and 55, and the Pegasus loop (Mizuno+22, ApJ 935, 97) using a component decomposition of the 21-cm HI line emission. We have now applied the same procedure to several local clouds. Under the assumption of an optically thin case, we found that narrow HI commonly shows larger emissivity than broad HI, confirming narrow HI to be optically thick. Gamma-ray emissivity normalizations of broad HI are found to agree with a model based on direct measurements of CRs. In this contribution, we present the analysis and results as well as discuss the implications for the Galactic CR/ISM.

Michael Moss (NASA/GSFC); Dr. Amy Lien, Dr. S. Bradley Cenko

The Durations and Fluences of High-z GRBs are Underestimated

In this study we investigated how Gamma-Ray Burst (GRB) prompt duration and fluence measurements are affected by increasing source redshift. We took bright GRBs observed by the Burst Alert Telescope on board the Neil Gehrels Swift Observatory (Swift/BAT) at redshifts z<1 and simulated what Swift/BAT would have observed if the GRBs were instead at higher redshifts (i.e., z>3), taking into account proper distance corrections and instrument response folding. We then used a Bayesian block algorit hm to measure the duration of the simulated GRBs. We found that almost all durations and fluences measured for simulated high-z simulated GRBs are shorter than their observed durations when corrected for time dilation and luminosity distance, respectively. The underestimations are due to low signal-to-noise burst emission being lost into the background, i.e., the so-called "tip-of-the-iceberg" effect. The amount of signal lost depends on light curve structure and brightness. We then compared the duration and fluence distributions of the GRBs we simulated at high-z to that of Swift/BAT GRBs observed at high redshifts and found that the two samples are consistent. These results imply that the GRB prompt duration and fluences of high-z GRBs observed by Swift/BAT and all other GRB instruments are all underestimations.

Yuya Nakamura (Nagoya University); GRAINE collaboration

GRAINE: Balloon-borne emulsion telescope project for the sub-GeV/GeV gamma-ray observation with a high angular resolution and a polarization sensitivity

The GRAINE project observes cosmic gamma-rays, using a balloon-borne emulsion-film-based telescope in the sub-GeV/GeV energy band. We aim to realize the observation with a high angular resolution (1 degree at 100MeV and 0.1 degree at 1GeV) and a polarization sensitivity by the very high spacial resolution of the emulsion film. Furthermore, the emulsion film has good scalability and we plan to observe with an aperture area of 10 m2 repeatedly. In here, we will report mainly about the previous balloon experiment (GRAINE2018) and the latest one (GRAINE2023). In GRAINE2018, we observed Vela pulsar with the highest angular resolution (~0.5 degree in >80MeV), and we established the cosmic gamma-ray observation by the emulsion film. In GRAINE2023, the aperture area of the telescope was 2.5 m2, which is about 6.6 times larger than that in GRAINE2018, and we started the scientific observation. The analysis of GRAINE2023 is ongoing now, and primary targets in it are Galactic center, especially in Galactic center GeV excess, and Vela pulsar for the cofirmation of the performance in the GeV energy band and the first trial of the polarization observation in the sub-GeV energy band.

Jorge Otero Santos (Instituto de Astrofísica de Canarias); Mireia Nievas Rosillo, Daniel Morcuende-Parrilla, David Sanchez, Axel Arbet-Engels; "LST Collaboration, CTAO

The most ancient VHE blazar yet: detection of FSRQ OP313 at z=0.997 with LST-1

The extragalactic very-high-energy (VHE, E>100 GeV) gamma-ray sky is dominated by BL Lacs; only 9 flat spectrum radio quasars (FSRQ) were known so far to emit in this band. The detection of OP 313 with the Large-Sized Telescope prototype (LST-1) of the Cherenkov Telescope Array Observatory (CTAO) in December 2023 adds the tenth and possibly most extraordinary quasar to the set. OP 313 is a record-breaking FSRQ in terms of distance, with a redshift of z=0.997, and had one of the most luminous long-lasting AGN outbursts ever recorded by the Fermi-LAT in its 16 years of operations. OP 313 opens one of the most interesting science cases for CTAO: the detection of high distance VHE sources in an Universe closer to its star formation history peak, and where galaxy mergers had a more predominant role at fueling AGN activity.

In this contribution, we will discuss the LST-1 detection of OP 313, its implications, and the deep multi-wavelength campaign developed to monitor the state of the source in multiple energy bands during its several months of activity. This result represents the first scientific discovery of LST-1, and a preview of what is coming with the construction of three more LSTs at the Roque de los Muchachos (La Palma), the northern site of CTAO.

Mehr Nisa (Michigan State University); Alejandra Granados, Rishi Babu; IceCube

Search for Astrophysical Neutrinos from 4FGL Galactic Plane Sources with the Pion Bump Signature

Gamma rays produced through the decay of neutral pions result in a distinctive "pion-bump" or "spectral break" around energies of 200 MeV. Detecting astrophysical neutrinos from regions that show the distinctive bump or spectral break would provide concrete evidence of cosmic-ray acceleration in the vicinity of such astrophysical sources. The IceCube Neutrino Observatory is a neutrino detector located at the South Pole, comprising a cubic-kilometer volume within the Antarctic ice. IceCube has observed a diffuse flux of neutrinos from the galactic plane, however, no point sources of astrophysical neutrinos from the Milky Way have been detected. A total of 56 sources in the 4FGL catalog by Fermi-LAT Collaboration have shown the spectral break signature. In this work, we will present a dedicated analysis of these 56 sources to look for astrophysical neutrinos using 13 years of IceCube data. We show that the analysis could advance our knowledge of potentially new acceleration sites and provide a direct probe of CR acceleration in the galaxy.

Pi Nuessle (George Washington/NASA Goddard); Judith Racusin and Nick White

A Novel GRB Progenitor Classifier Based on Fermi-GBM Prompt Emission Properties

Of the two known types of gamma-ray burst (GRB) progenitor systems, most neutron star mergers have been associated with GRBs with a duration (T90) shorter than 2 seconds or some instrument-dependent value. Meanwhile, massive stellar collapsars were be found to be longer. Considering several recent mergers longer than five seconds, and a collapsar shorter than one second, we created a novel GRB progenitor classifier trained on detected progenitors with associated supernovae, kilonovae, and large host galaxy offsets. Using a support vector machine (SVM), a small number of known progenitors in GRB catalogs are used to predict the progenitors of all the bursts in our sample. Our parameterization is based on previously established correlations using the prompt duration, peak energy of the prompt spectrum, and prompt fluence. Based on covariance testing from Fermi-GBM, Swift-BAT, and Swift-XRT data, we find that prompt features alone can create a statistical technique providing a probabilistic determination of a GRB progenitor, reflecting the new understanding that mergers and collapsars exist in both "long" and "short" GRB populations.

Nicola Omodei (Stanford University and KIPAC); Elisabetta Bissaldi, Philippe Bruel, Niccolo' Di Lalla, Roberta Pillera; on behalf of the Fermi LAT collaboration

GRB 221009A: the B.O.A.T that Shines in Gamma-rays

GRB 221009A, also known as the B.O.A.T, was observed by dozens of space- and ground-based observatories, including both instruments onboard the Fermi telescope: the Large Area Telescope (LAT) and the Gamma-Ray Burst Monitor (GBM). The triggering pulse, detected by Fermi-GBM, was followed by a prompt phase lasting a few hundred seconds, and by an extended emission which was detected by Fermi-LAT for over two days. In this presentation, we summarize the highlights from the LAT analysis of this extraordinary event, including the effort to recover data during the extremely intense hard X-ray and soft gamma-ray flux, which compromised the LAT data quality for about one minute. Furthermore, we found that the high-energy events detected by the LAT cannot have a Synchrotron origin but, during the prompt emission, they are probably associated with an additional Synchrotron Self Compton (SSC) component, which is also responsible for the TeV emission detected by the Large High Altitude Air Shower Observatory (LHAASO). Late-time high-energy events are instead harder to explain as products of SSC or TeV electromagnetic cascades, which raises questions regarding their origin. Overall, GRB221009A stands out compared to other Fermi-LAT GRBs, indicating that it is an exceptionally rare event.

Tyler Parsotan (NASA GSFC); Craig Markwardt, David Palmer, Sibasish Laha, Amy Lien; Swift BAT

A Comprehensive Python Pipeline for Swift BAT Data Analysis

The Swift Burst Alert Telescope (BAT) is a coded aperture gamma-ray instrument with a large field of view that primarily operates in survey mode when it is not triggering on transient events. Survey data comprises > 90% of all of BAT data by volume and allows for the tracking of long term light curves and spectral properties of cataloged and uncataloged hard X-ray sources between 14 and 195 keV. Until now, the survey dataset has not been used to its full potential due to the complexity associated with its analysis and the lack of easily usable pipelines. Here, we introduce the BatAnalysis python package which provides a modern, open-source pipeline to process and analyze BAT survey data. BatAnalysis allows members of the community to use BAT survey data in more advanced analyses of astrophysical sources. Additionally, the package now allows for the convenient analysis of BAT event data, unlocking the full potential to produce spectra and light curves with arbitrary time bin and energy bins. The BatAnalysis package allows for ~20 years of BAT survey and event data to be used in joint studies of a variety of astrophysical sources.

Parshad Patel (George Mason University); Justin D. Finke

Lorentz Invariance Violation with Absorption of Astrophysical Gamma-rays by Solar Photons

We explore in detail the absorption caused by astrophysical gamma-ray photons interacting with solar photons to produce electron-positron pairs. This absorption is greatest for gamma-ray sources at small angular distances from the Sun. We also highlight the role of subluminal Lorentz invariance violation (LIV) for changes to this absorption, including modifications to the absorption cross-section threshold. We show for the first time that subluminal LIV can lead to increases or decreases in the gamma-ray optical depth compared to the non-LIV case, contingent upon the energy of the incoming gamma-ray photon. We find that the non-LIV case can be probed within 37 years of LHAASO observations at 1 TeV and 3 sigma. We also show that, at least in principle, LIV can be probed with this effect with observations of gamma-ray sources near the Sun at >20 TeV by HAWC or LHAASO, although a measurement will be extremely difficult due to the small size of the effect and the cosmic ray background.

Pablo Penil (Clemson University); A. Domínguez, S. Buson, M. Ajello

Multiwavelength Analysis of Fermi-LAT Blazars Exhibiting Hints of Periodicity

Blazars are well-known for their highly variable gamma-ray emissions, a characteristic that manifests across all wavelengths. This variability can span from minutes to several years. In our investigation, we utilize the initial 12 years of data collected by the Fermi Large Area Telescope, supplemented by multiwavelength archival data from various observatories operating in radio, optical, and X-ray bands. Our aim is to comprehensively explore potential periodic emissions from 24 blazars previously identified as periodic candidates. As a result, we discover that 7 of these blazars exhibit consistent periodic patterns across their multiwavelength data with a significance of ≥3 sigma. Furthermore, we identify a long-term ~10-year rising trend in the light curves of PG 1553+113, which we utilize to deduce potential constraints on the binary black hole hypothesis.

Jeremy Perkins (NASA/GSFC); BurstCube Team

BurstCube: A CubeSat for Gravitational Wave Counterparts

BurstCube is a 6U CubeSat comprised of 4 CsI scintillators read out by arrays of SiPMs designed to detect gamma-ray bursts (GRBs) and other short gamma-ray transients and enable multi-messenger astrophysics. Observations of coincident GRBs and gravitational waves from compact binary mergers provides a wealth of science, and BurstCube is operating during the O4b observing run. It was launched in March 2024 and deployed into Low Earth Orbit from the International Space Station on April 18, 2024, with science operations beginning after a commissioning period. Like many other GRB detectors on mission scales from CubeSats to Fermi, BurstCube will detect GRBs, increase sky coverage, and inform the follow-up efforts of both space and ground-based observatories. The onboard instrument triggering algorithm autonomously enables rapid downlink of data using the Tracking and Data Relay Satellites (TDRS) - BurstCube is the first CubeSat to do so. The alerts are distributed via the General Coordinates Network (GCN) within minutes of detection, and data is publicly available via HEASARC immediately upon processing with open-source analysis software. This presentation will describe the current status of the mission and early science results.

Alexander Philippov (University of Maryland, College Park)

Pulsar magnetospheres and their radiation

In this talk, I will present the most up-to-date view of pulsar magnetospheres and their radiation as achieved with numerical kinetic plasma simulations. Specifically, I will highlight the role of the reconnecting current sheet in shaping the observed high-energy emission seen by Fermi.

Nabin Poudyal (GRAMS, Northeastern University); For the GRAMS collaboration; GRAMS

Enhancing Astrophysical MeV Gamma-Ray Detection with GRAMS

The Gamma-Ray and AntiMatter Survey (GRAMS) is a NASA suborbital mission within the Physics of the Cosmos program. Its primary aim is to detect astrophysical gamma-rays in the MeV range while also possessing the unique capability to explore dark matter through antimatter detection. Gamma-ray surveys are important for understanding multi-messenger and time-domain astronomy, enabling exploration of the universe's most potent events such as supernovae and neutron star mergers. They also shed light on phenomena like gamma-ray bursts and variable stars, providing insights into their temporal patterns and underlying physical processes. Despite the significance of MeV gamma-rays, they have been relatively under-explored due to limitations in detector technology in accurately reconstructing Compton events. Our proposed solution was a large-scale liquid argon (LAr) Time Projection Chamber (TPC), which serves as both a Compton camera and calorimeter, facilitating precise measurement of incident gamma-ray energy and direction to the source. This advancement is made possible by the improved energy and angular resolution of the GRAMS LArTPC detector in the MeV energy region. I will provide an update on the current status and progress towards the first prototype balloon flight with a small-scale LArTPC (pGRAMS) scheduled for 2025/2026 with the objective of accurately reconstructing gamma-ray events and tracking charged particles.

Rachel Procter-Murphy (University of Maryland/IceCube); Bennett Brinson, Karlijn Kruiswijk, Jessie Thwaites, Nora Valtonen-Mattila

The IceCube search for neutrinos from GRB 221009A

GRB221009A is the brightest gamma-ray burst (GRB) ever observed, providing a unique opportunity to search for associated neutrino emission. We use the IceCube Neutrino Observatory to perform a model independent search for neutrinos across a wide range of energy using several time windows coinciding with the precursor, the prompt phase, and the afterglow of this GRB. A search for MeV neutrinos utilizes photo-multiplier rate scalers, normally used to search for galactic supernova neutrinos. A search for 0.5-5 GeV neutrinos utilizes near-threshold events from the DeepCore subdetector, which lacks directional information, but is sensitive to an excess of events. Searches for 10 GeV - 1 TeV and > TeV neutrinos include timing and directional information. We place upper limits on several models across the range for predicted neutrino emission. An IceCube realtime alert system for neutrino emission from gamma-ray bursts is currently being developed to respond with a targeted search to future events like this one with low-latency.

Pazit Rabinowitz (Washington University in St. Louis); VERITAS

An unbiased survey of high-frequency-peaked BL Lac objects by VERITAS

More than 50 high-frequency-peaked BL Lac objects (HBLs) have been detected by ground-based TeV gamma-ray observatories, making them the dominant population of extragalactic sources at energies above 0.1 TeV. Between 2019 and 2022, the VERITAS observatory has conducted an observing campaign of 36 X-ray selected HBLs, aiming to produce the first unbiased survey of HBLs at TeV energies, leading towards a luminosity function for these sources. The VERITAS HBL sample consists of 21 known TeV sources and 15 blazars without previously reported TeV emission. The fluxes of HBLs are often reported only during high-flux states, biasing our understanding of the properties and duty cycle of these sources towards flares. The goal of this survey is instead to find an unbiased estimate of the average TeV flux for each source, while also searching for detections of new TeV blazars which have not been previously observed. The newly completed results of the full survey will be presented, as well as their implications for understanding the intrinsic properties of TeV-emitting HBL sources.

Garima Rajguru (Clemson University); Lea Marcotulli and Marco Ajello

Mapping the Cosmic Evolution of Fermi Blazars

Blazars are a class of active galactic nuclei that have their extreme jets directed at us. The evolution of blazars through cosmic time has been investigated for over 30 years, but the results have been elusive due to small sample sizes and limited redshift range. The Fermi Gamma Ray Space telescope provides the largest catalog of gamma-ray blazars and their properties, such as redshift (z). This has enabled us to obtain a sample almost 6 times larger than the largest dataset ever used to study the evolution of blazars, along with a larger redshift range (z~4). With over 1000 BL Lacertae objects and 500 flat-spectrum radio quasars we can study the evolution of the two source classes, any connection between them and their contribution to the extragalactic gamma-ray background. In this talk, I will present the latest results from our analysis.

Soebur Razzaque (University of Johannesburg and the George Washington University); Luyanda Mazwi, Lutendo Nyadzani

On non-detection of Gamma-Ray Bursts in three compact binary merger events detected by LIGO

The joint detection of gravitational wave (GW) event GW170817 and the short GRB event GRB 170817A marked the beginning of GW multi-messenger astronomy and confirmed that binary neutron star mergers are progenitors of at least some short GRBs. An estimated joint detection rate of 0.3-1.7 per year between the LIGO Hanford, LIGO Livingston and Virgo GW network at design sensitivity, and the Fermi Gamma-ray Burst Monitor was predicted. However, to date, the GW170817/GRB 170817A joint detection has been the only event of its kind. Taking into account that GRBs are narrowly beamed and are collimated perpendicular to the orbital plane of the binary system, we propose that previous mergers involving neutron stars, were orientated such that observation of the GRB along this narrow jet was not possible. To support this hypothesis we have estimated the inclination of the binary systems for previously detected Binary Neutron Star (BNS) and Black Hole Neutron Star (BHNS) mergers through GW analysis, using BILBY, a Python based Bayesian inference library, to estimate the inclination of the BNS events GW170817 and GW190425, and the BHNS events GW190917_114630 and GW200115_042309. The results obtained in this study all show that these binaries had large inclinations with respect to the line of sight from Earth. Based on these results, we discuss implications for the joint GW-GRB detection rate.

Valentina Richard Romei (Université Grenoble Alpes - CNRS); Benoît Cerutti

Enhanced high-energy emission in a pulsar wind interacting with a companion

Pulsar winds have been shown to be preferred sites for particle acceleration and high-energy synchrotron radiation falling in the γ-ray energy band. Several studies have been conducted in order to better characterize the general structure of such relativistic plasmas in isolated systems. However, most of the galactic millisecond pulsars find themselves in binary systems. In this work, we present the first Particle-in-cell (PIC) simulations of both the pulsar magnetosphere and its wind interacting with an unmagnetized spherical companion. We resort to 2D equatorial PIC simulations, that have the advantage of simulating both the magnetosphere's global dynamics and the microphysical processes responsible for particle acceleration and radiation. This work considers a generic case that could be applied to various companions including planets, asteroids, white dwarfs, or even neutron stars. We investigate the rearrangement of the pulsar wind in the presence of the companion, for different companion separations and sizes. In particular, we focus on the additional particle acceleration and the related non-thermal electromagnetic emissions that stem from this interaction. Our results show evidence of an enhanced conversion of Poynting flux into particle acceleration, via forced reconnection in the outflowing wind near the companion. Hence, the high-energy synchrotron radiation is also amplified and takes the form of an orbital-modulated γ-ray hollow cone of light. We do not exclude long-period radio transient counterparts, that would be of significant interest, especially in the light of the recently discovered galactic long-period radio transients.

Samuele Ronchini (PennState University)

Current and future role of the Fermi Gamma-ray Space Telescope for multi-messenger astronomy

Since the gravitational wave (GW) astronomy started to flourish, the Fermi telescope had always a crucial role for the detection of the high-energy counterparts of GWs. With the detection of a short GRB in coincidence with GW170817, we experienced the vast and multi-disciplinary impact of a joint gamma-ray/GW detection. In this talk I will review all the possible scientific implications of joint GRB/GW detections, underlying the steps forward already achieved and the questions still open. Particular emphasis will be given to the cooperation of Fermi with other space telescopes that operate in the X-ray/gamma-ray domain, and the relative impact for the detection, localization and characterization of the electromagnetic (EM) counterparts of GWs. In the future perspective, I will try to show how instruments like Fermi will lead the multi-messenger landscape, especially when the third generation of GW detectors, such us Cosmic Explorer and Einstein Telescope, will start to be operative. In the 3G GW era, the systematic detection of the gamma-ray emission from compact binary mergers will be the only way to confirm the EM-GW association at high redshift, with a deep relevance for several fields, such as cosmological studies, fundamental physics, GRB physics, population studies, as well as for investigating the connection between merger-driven GRBs and their progenitors.

Matt Roth (Los Alamos National Laboratory); HAWC

TeV halo physics with HAWC

We present a novel modeling effort for pulsar TeV halos that incorporates a detailed treatment of cosmic ray transport and emission processes around the source, while leveraging observations from the High Altitude Water Cherenkov (HAWC) observatory to constrain model parameters. Our model includes the full palette of cooling and emission processes, and accounts for ballistic and diffusive transport. By using HAWC observations to study their energy-dependent morphology, we seek to better understand cosmic ray propagation and the environmental conditions around middle aged pulsars which drive the formation and evolution of TeV halos. This methodology yields broadband spectra from the radio to gamma-ray energies and can ultimately be applied to various source classes to study the physics of extended emission from cosmic ray cooling across the electromagnetic spectrum.

Benjamin Safdi (University of California, Berkeley)

The Impact of the Fermi LAT on the Search for Particle Dark Matter

Weakly interacting massive particle (WIMP) dark matter remains one of the most promising dark matter (DM) candidates. One of the most promising avenues for WIMP identification is through the gamma-ray signatures associated with WIMP annihilation in the Milky Way and other galaxies. I will review how the Fermi LAT has changed the landscape of WIMP DM and particle DM more broadly by ruling out much of what was previously considered to be some of the most promising parameter space. On the other hand, I will discuss how many of the canonical benchmark models for WIMPs, such as the higgsino that emerges in supersymmetry, remain un-probed or barely probed by Fermi, though this could change in the coming years as I will discuss.

Kartick Sarkar (Indian Institute of Technology Kanpur, India)

Fermi Bubbles: past, present, and the future

The last decade of studying the diffuse gamma-ray emission from our Galaxy using the Fermi Telescope has advanced our understanding of the high energy processes in galactic winds, particularly the Fermi Bubbles. The gamma-ray observations have been complimented by observations in X-ray, radio, and UV bands. Together, they represent a complex view of the energetic processes in the Galactic center. Several theoretical models have been proposed to explain these features but with no consensus about their origin. In this talk, I will review the last decade of studying the Fermi bubbles and present the current understanding of these bubbles. I will also discuss how we can use the Fermi telescope to strengthen our understanding of these bubbles.

Pablo Saz Parkinson (UCSC); Alessio Fiori, Massimiliano Razzano, Alice Harding; LAT Collaboration

X-ray and Gamma-ray Observations of the mode-changing gamma-ray pulsar PSR J2021+4026

PSR J2021+4026 is a young, energetic, radio-quiet gamma-ray pulsar in the Gamma-Cygni supernova remnant that also shows thermal X-ray pulsations. Unique among the gamma-ray pulsar population, J2021 undergoes quasi-periodic mode changing episodes (e.g. in 2011, 2018) during which it shows correlated gamma-ray emission and spin-down variability. A shift in the gamma-to X-ray pulse profile alignment during one episode suggests that these may affect different regions of the pulsar magnetosphere differently. We will report on the latest findings from our X-ray and Gamma-ray monitoring observations of this unique system, and discuss the possible theoretical interpretation of our observations.

Jeremy Schnittman (NASA/GSFC)

The Distribution and Annihilation of Dark Matter Around Black Holes

We consider a simple model for dark matter self-annihilation where the cross section depends on energy, analogous to many standard model particle interactions. Since the only known way to accelerate dark matter is through gravitational forces, we focus on the collisions of particles orbiting around black holes. To do so, we use a Monte Carlo code to integrate the geodesic orbits of test particles around Kerr black holes, generating a distribution function of both bound and unbound populations of dark matter particles. From this distribution function, we calculate annihilation rates and observable gamma-ray spectra. We find that for rapidly spinning black holes, the collisional Penrose process can reach efficiencies of >600%, leading to a high-energy tail in the annihilation spectrum. Furthermore, the high particle densities and large proper volume of the region immediately surrounding the horizon ensures that the observed flux from these extreme events is non-negligable.

Stephen Sclafani (University of Maryland); IceCube

Correlation of High Energy Neutrinos with Fermi-LAT diffuse emission templates

The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector located in the geographic South Pole, that detects astrophysical neutrinos at TeV to PeV energies. In 2023, IceCube reported 4.5sigma correlation between IceCube cascade neutrinos and a spatial template of diffuse galactic emission from Fermi-LAT data. This emission occurs when cosmic rays interact with the dust and gas of the galactic plane and is the neutrino counterpart to the observed gamma-ray diffuse flux. This result was enabled by the use of machine learning to improve the selection of cascade events, which have less background in the southern sky, where emission is stronger. This represents the first observation of the Milky Way Galaxy in high-energy neutrinos. Since that observation IceCube has found consistent results with other data streams.

Lorenzo Scotton (The University of Alabama in Huntsville); Fermi-GBM Team

Fermi-GBM follow-up of gravitational waves during O4

The joint detection of gravitational waves (GWs) and electromagnetic radiation from the binary neutron star (BNS) merger GW170817 opened a new window in astrophysics. This joint detection was the first of its kind and confirmed BNS mergers as a progenitor class of short gamma-ray bursts (GRBs). The on-board detection of GRB 170817A, the gamma-ray counterpart of GW170817, has made Fermi-GBM a leading instrument in multimessenger astrophysics. Fermi-GBM monitors continuously the entire unocculted sky and it is sensitive to gamma-ray radiation between 8 keV and 40 MeV. The significant rate of on-board GRB detections over more than 15 years of operations sets Fermi-GBM as an ideal platform to search for gamma-ray counterparts to GWs in real time. Moreover, Fermi-GBM sub-threshold ground techniques are used to follow-up GWs. This presentation will cover the status and results of the Fermi-GBM searches over the ongoing fourth LVK observing run.

Vidushi Sharma (NASA GSFC/UMBC); Judith Racusin, Leo Singer; GCN Team

General Coordinates Network (GCN): New GCN Notices and Machine Readability of Circulars

General Coordinates Network (GCN) is a collaborative platform operated by NASA to facilitate the rapid sharing of alerts and communication within the astronomy research community regarding high-energy, transient, and multi-messenger phenomena. GCN has introduced the GCN Unified Schema, which acts as the foundation for the new GCN Notices that are exclusively streamed through Kafka and employ a JSON format for easy machine readability and accessibility through an open-source repository. Unified Schema is designed with a set of core schemas to define common properties across various missions, while also allowing flexibility for the inclusion of instrument-specific properties. For GCN Circulars, Neural Topic Modeling with BERTopic is employed to track the progression of different observation types over the three decades of GCN. The large-language model has been utilized to extract redshift information in tabular form and has the potential for further use in extracting useful information from the database. In this presentation, we will describe how to produce the new GCN Notices, inclusion of new missions BurstCube, Einstein Probe, and SVOM, and outline our vision for its future growth as a community resource.

Dimitrios Skiathas (SURA, NASA Goddard Space Flight Center, University of Patras); Costantinos Kalapotharakos, Zorawar Wadiasingh, Demos Kazanas, Alice Harding, Paul Kolbeck

Exploration of electromagnetic outflows of inspiraling binary neutron stars

Broadband electromagnetic emissions originate from the magnetospheres of neutron stars. In the case of binary systems involving those objects, emissions are fueled by the dynamic interaction of their magnetospheres during the inspiraling motion of the neutron stars. These emissions, produced prior to the merger, could serve as precursor signals to gravitational wave events from binary neutron star mergers. Such signals provide crucial information about the nature of the merging neutron stars, assisting in follow-up observations and their localization. Additionally, emissions redirected after interacting with interstellar gases and observed as echoes could potentially contribute to afterglow rebrightenings with a time delay. Understanding the patterns and characteristics of these pre-merger emissions requires a comprehensive exploration of the magnetospheres' global electromagnetic field structure. Our study employs numerical simulations to thoroughly explore the multidimensional parameter space that involves the relative strengths and directions of the dipole moments, their direction relative to the orbital axis, and the star spin periods. We calculate the Poynting flux and the gamma-ray emission patterns of the system in different cases, finding diverse, non-uniform, and non-symmetric distributions. Additionally, we investigate how the Poynting fluxes depend on the varying orbital angular frequency over time, revealing the diverse rates at which the magnetospheres lose energy. Finally, we discuss the potential implications of asymmetric outflows on the orbits and magnetic alignments of the neutron stars due to the corresponding kicks and torques. We also explore the expected echoes during the inspiral evolution, highlighting their potential contributions to our understanding of these complex systems.

Deheng Song (Yukawa Institute for Theoretical Physics, Kyoto University); Christopher Eckner, Chris Gordon, Francesca Calore, Oscar Macias, Kevork N. Abazajian, Shunsaku Horiuchi, Manoj Kaplinghat, Martin Pohl

Robust inference of the Galactic center excess spatial morphology

The nature of the Fermi-LAT Galactic Center Excess (GCE) has remained a puzzle for over 15 years, with ongoing debates about its properties, particularly its spatial morphology. This talk will present a comprehensive analysis of how the estimated spatial morphology of the GCE depends on models for the Galactic diffuse emission, with a focus on the extent to which the Galactic plane and point sources are masked. Our main aim is to compare a spherically symmetric morphology - potentially arising from the annihilation of dark matter particles - with a boxy morphology - expected if faint unresolved sources in the Galactic bulge dominate the excess emission. We show that recent claims favouring a dark matter-motivated template for the GCE rely on a specific Galactic bulge template, which performs worse than other templates for the Galactic bulge. We find that a non-parametric model of the Galactic bulge derived from the VVV survey results in a significantly better fit for the GCE than dark matter-motivated templates. This result holds true regardless of whether a GALPROP-based model or a more non-parametric ring-based model is used to describe the Galactic diffuse emission. Our conclusion remains robust even when additional freedom is introduced in the background models, allowing for non-parametric modulation of the model components and substantially improving the fit quality.

Tinn Thongmeearkom (JBCA, the University of Manchester); Colin J. Clark, Rene P. Breton et at; TRAPUM

Finding Gamma-ray Redback Pulsars with TRAPUM on MeerKAT

Redbacks are compact-orbit binary star systems with a millisecond pulsar and a low-mass irradiated companion. These systems exhibit properties that can be used to study binary evolution, pulsar winds, and neutron star mass measurements. Over the years, Fermi-LAT has detected numerous unidentified gamma-ray sources. Some of these identified sources have pulsar-like gamma-ray spectra. Following this, many projects have monitored these sources and found some high-confidence redback candidates based on their optical and/or X-ray light curves, but these remained unconfirmed in the absence of detected radio pulsations. Attempting to confirm these systems as millisecond pulsar binaries, we conducted a deep survey of six redback candidates with MeerKAT at L-band and UHF as part of the Fermi sources working group of TRAPUM. In this talk, we will present three newly discovered millisecond pulsars. One of them has extraordinary eclipsing behavior. We will also present the results of our radio and gamma-ray timing campaigns, which provide orbital parameters for pulsar mass measurements. Finally, we will explain the importance of Fermi-LAT for finding pulsars in the past and future.

Lucia Tian (NASA Goddard Space Flight Center); Israel Martinez-Castellanos, Hugo Ayala, Nicola Omodei, Niccolò Di Lalla; "HAWC Collaboration, Fermi-LAT Collaboration

Joint Analysis of GRB 221009A Late-Time Emission Using Fermi-LAT and HAWC

The recent gamma-ray burst (GRB) 221009A was the brightest GRB ever observed, producing photons of record TeV energies as well as an exceptionally extended afterglow across the electromagnetic spectrum. At 8.0 hours post-trigger, the burst position finally began transiting the High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory's field of view, but HAWC (sensitive to energies of 100 GeV - >100 TeV) found no significant detections in searches on timescales of seconds to hours. At 1.3 hours into this transit, however, the Fermi mission's Large Area Telescope (LAT) (sensitive to 20 MeV - several hundred GeV) detected a spatially consistent event of ~400 GeV. This event would be the LAT's highest-energy GRB photon ever recorded if associated with the burst, with a ~3e-5 probability (~4σ) of being a background fluctuation from an unresolved source or diffuse emission. To increase sensitivity, account for small samples and non-detections, help reconcile apparent data discrepancies, and produce tighter constraints on spectral parameters, one can combine the Fermi-LAT and HAWC observations into a simultaneous, joint spectral fit. Using the Multi-Mission Maximum Likelihood (3ML) Framework and a new 3ML plugin that enables HAWC transient analyses, we obtain joint LAT-HAWC fits over the extended range of ~100 MeV - 10 TeV. We find the hypothesis of a short-duration flare or rebrightening to be unlikely, instead focusing on the possibility that the detected emission is an extension of the afterglow seen by LHAASO. This talk will describe the joint-fit results over the late-time period as well as the model's implications for GRB 221009A.

Aaron Trigg (Louisiana State University); Oliver J. Roberts, Michela Negro, Dmitry S. Svinkin, Zorawar Wadiasingh, Nelson L. Christensen, Igor Andreoni, Eric Burns, Matthew G. Baring, Michael S. Briggs, Niccol\`o Di Lalla, Dmitry Frederiks, Vladimir Lipunov, Nicola Omodei, Anna V. Ridnaia, Peter Veres, Narayana Bhat, George Younes, Brendan O'Conner

Unveiling Extragalactic Magnetar Giant Flares with Fermi GBM: A Comparative Study of GRB 231115A and GRB 180128A

Magnetars, a subset of neutron stars, exhibit intense, short-lived (~ 10^4 yr) magnetic fields ranging from 10^14 to 10^15 Gauss. They display a diverse range of high-energy electromagnetic phenomena. The most energetic among these, with an isotropic-equivalent energy output (E_iso) of approximately 10^44 to 10^47 erg, is the magnetar giant flare (MGF). As of early 2023, only seven such events had been identified, with three observed in the Milky Way and Large Magellanic Cloud. Until recently, GRB 200415A stood as the sole MGF candidate detected by Fermi-GBM. However, this past November, two new MGFs were identified - GRB 180128A from NGC 253 and GRB 231115A from M82. The newest extragalactic MGF candidate, GRB231115A, is the first promptly identified and localized MGF ever. These constitute the second MGFs identified in their host galaxies, which have otherwise only been observed to occur within the Milky Way itself. Comparing new and breaking results from this latest detection with GRB 180128A enables a more comprehensive understanding of the energies, rates, emitting regions, and physical mechanisms of MGFs. This side-by-side analysis of these two MGFs allows us to gain insights into these extreme events. Our presentation will showcase these recently identified MGFs, contributing to the evolving understanding of this enigmatic astrophysical phenomenon.

Marco Turchetta (Norwegian University of Science and Technology); Manuel Linares, Karri Koljonen, Paulo Miles Páez; "LOVE-NEST team, funded by the ERC consolidator grant

Discovery of two new spider pulsar candidates in previously unassociated Fermi-LAT sources

Since its launch in 2008, Fermi-LAT is uncovering a new nearby population of compact binary millisecond pulsars, also known as "spiders". Expanding the currently known spider population holds the key to find the fastest-rotating and most massive neutron stars, and to understand the effects of irradiation on the companion star by the relativistic pulsar wind. Although most of these systems are found from radio follow-up of Fermi sources, searching for the variable optical emission of their companion can reveal a hidden population of spiders that are highly obscured in radio wavelengths.

We present a systematic optical photometric survey of a selected group of pulsar-like Fermi-LAT unidentified sources. We report the discovery of two variable optical counterparts to 4FGL J0736.9-3231 and 4FGL J2117.9+3729, which we identify as new spider candidates. Their optical light curves show flux modulation larger than 0.2 mag in the sub-day period range and effective temperatures of 5000-6000 K, all properties consistent with the known sub-type of redback binary millisecond pulsars. These results will not only facilitate future radio observations of these two candidates, but will also allow us to estimate their neutron star masses via optical light curve modelling. Optical searches are crucial to identify radio-obscured spiders, especially for those systems with low orbital inclinations or weak/lacking irradiation like 4FGL J0736.9-3231 and 4FGL J2117.9+3729.

Janeth Valverde (UMBC / NASA GSFC); The Fermi-LAT Collaboration

A history of extreme explosions: GeV flares in Fermi-LAT variable sources

The study of temporal variability, particularly flares, is pivotal to understanding the physics that underlies those variations, many of which are not well understood. These variations could be tied to unobservable activity in accreting objects, to the precession of collimated relativistic plasma, the nature of the parent particle population, changes in the field strength or particle acceleration in the emission zone.

Studying the variable emission from gamma-ray steady sources can be challenging, since generating mission-long well-sampled light curves is computationally expensive. In this contribution, we make use of the publication-quality, mission-long, continuously updated Fermi-LAT Light Curve Repository data to circumvent this issue, validate it, and extract variability features such as flare duration, symmetry, and duty cycle. We present our findings on the characteristics of these flares according to the source type.

Thomas Venville (Research School of Astronomy and Astrophysics, Australian National University); Oscar Macias (San Francisco State University), Roland Crocker (Research School of Astronomy and Astrophysics, Australian National University), Alan Duffy (Swinburne University of Technology), Thor Tepper Garcia (University of Sydney)

A search for dark matter annihilation from the Sagittarius Dwarf and Stream

While Dwarf Spheroidal galaxies are amongst the most promising targets for indirect dark matter (DM) detection experiments, extensive past searches have not found convincing evidence for DM annihilation in these objects. However, in Crocker et al. (2022) a gamma-ray signal was detected from the core of the Sagittarius Dwarf Galaxy (Sgr) at 8.1 sigma statistical confidence. In subsequent works we assess whether this emission could be a DM signal (Venville et al. 2024), then undertake a comprehensive re-analysis of Fermi LAT gamma-ray data to search for significant gamma-ray emission in the Sagittarius Stream (Venville et al, in prep). We overcome the difficulty of modelling the tidally disrupted Sgr and Sagittarius Stream DM distributions by computing the expected J-factor distribution utilising a hydrodynamic simulation of the formation of the Sgr and the Sagittarius Stream. We find the predicted J-factor value for Sgr, J_Sgr = 1.48 × 10^10 M_⊙^2 kpc^-5 (6.46 × 10^16 GeV cm^-5), would require a DM annihilation cross section incompatible with existing constraints in order for DM annihilation to explain the observed gamma-ray emission. Our results imply that most past studies have overestimated the DM density of Sgr on small scales, with potential implications for other indirect DM detection searches using dwarf spheroidal galaxies. The analysis of the Sagittarius Stream also failed to detect any significant gamma-ray emission attributable to DM annihilation.

Peter Veres (University of Alabama in Huntsville); Wade Duvall, Adam Goldstein, Michael Briggs, Eric Grove

The Fermi Gamma-ray Burst Monitor as a Polarimeter

We present polarimetric analysis of the bright GRB 180720B using the Fermi-Gamma-ray Burst Monitor (GBM). Polarized gamma-rays will exhibit a characteristic pattern when scattering off the atmosphere. GRB 180720B occurred close to the zenith of Fermi resulting in a large atmospheric scattered gamma-ray flux. We compare the measured photon counts in GBM with extensive simulations of polarized beams to derive the most probable polarization degree (PD) and angle (PA). For the entire GRB, we find PD = 72 +/- 27%. Interestingly, the PA value is broadly consistent with an early optical PA measurement by the Kanata telescope, starting shortly after the end of the prompt emission. The consistency of PAs lends support for this method. The relatively high polarization degree (albeit with large uncertainties) agrees with similar past measurements suggesting that some GRBs might be highly polarized.

Zorawar Wadiasingh (UMD College Park / NASA GSFC); Mnju Sim, Hongjun An

MSPs are likely PeV accelerators as Demonstrated by Fermi-LAT

We investigated the multiband emission from the pulsar binaries XSS J12270-4859, PSR J2039-5617, and PSR J2339-0533, which exhibit orbital modulation in the X-ray and gamma-ray bands. We constructed the sources' broadband spectral energy distributions and multiband orbital light curves by supplementing our X-ray measurements with published gamma-ray results, and we modeled the data using intrabinary shock (IBS) scenarios. While the X-ray data were well explained by synchrotron emission from electrons/positrons in the IBS, the gamma-ray data were difficult to explain with the IBS components alone. Therefore, we explored other scenarios that had been suggested for gamma-ray emission from pulsar binaries: (1) inverse-Compton emission in the upstream unshocked wind zone and (2) synchrotron radiation from electrons/positrons interacting with the kilogauss magnetic field of the companion. Scenario (1) requires that the bulk motion of the wind substantially decelerates to ~1000 km s-1 before reaching the IBS for increased residence time, in which case the formation of a strong shock is untenable, inconsistent with the X-ray phenomenology. Scenario (2) can explain the data if we assume the presence of electrons/positrons with a Lorentz factor ~ 10^8 (~0.1 PeV) that pass through the IBS and tap a substantial portion of the pulsar voltage drop. These findings raise the possibility that the orbitally modulating gamma-ray signals from pulsar binaries can provide insights into the flow structure and energy conversion within pulsar winds and particle acceleration nearing PeV energies in pulsars. These signals may also yield greater understanding of kilogauss magnetic fields potentially hosted by the low-mass stars in these systems.

Sarah Wagner (University of Wuerzburg); Jeffrey D. Scargle, Greg Madejski on behalf of the Fermi-LAT Collaboration, Andrea Gokus, and Krzysztof Nalewajko; Fermi-LAT Collaboration

High-energy variability of the gravitationally lensed blazar PKS 1830-211

The production site and process responsible for the highly variable high-energy emission from blazar jets are still debated. Gravitational lenses can be used as microscopes to investigate the nature of such sources. We study the broad-band spectral properties and the high-energy variability of the gravitationally-lensed blazar PKS 1830-211, for which radio observations have revealed two images, to put constraints on the structure and relevant physics of its jet. We utilize Swift-XRT, NuSTAR, and Fermi-LAT observations from 2016 and 2019 to compare periods of low activity and high activity in PKS 1830-211. Short-timescale variability is elucidated with an unbinned power spectrum analysis of time-tagged NuSTAR photon data. Regarding the gravitationally-induced time delay in the gamma-ray light curve observed with Fermi-LAT, we develop a metric optimization method yielding a delay of 22.4 +/- 5.7 days consistent with the value obtained by our auto-correlation approach, 21.96 +/- 0.30 days, both of which being constant over time. When comparing the 2016 and 2019 datasets, the X-ray part of the SED is remarkably constant in comparison to the dramatic change in the gamma-rays. The X-ray and gamma-ray parts of the SED can be fitted with a single component resulting from Comptonisation of infrared emission from the dusty torus, with different gamma-ray states arising solely due to a shift of the break in the electron energy distribution. The detection of a consistent lag throughout the whole light curve suggests that the the gamma-rays originate form a persistent location in the jet.

Xiaojie Wang (Michigan Technological University); HAWC

Microquasars Detections with HAWC: Exploring the Intriguing V4641 Sgr

As the stellar analogs of extragalactic quasars, microquasars have been recognized as powerful Galactic particle accelerators that emit gamma rays exceeding multi-TeV energies since the first detection of SS 433. Interest in these objects has surged over the last decade. Recently, several have been observed emitting gamma rays above tens of TeV. The High Altitude Water Cherenkov (HAWC) observatory previously reported gamma-ray emissions with a median energy of 25 TeV from SS 433, demonstrating that gamma-ray binaries can accelerate particles to over 100 TeV within their jets. Utilizing approximately 2400 days of data and improved reconstruction algorithms (Pass5), HAWC detected emissions above 20 TeV from two additional binaries, V4641 Sagittarii and LS 5039. V4641 Sgr, in particular, has been detected with a significance greater than 9 sigma. It features a 6.4 solar mass black hole and a main sequence B-type companion of about 2.9 solar masses, notable for its super-Eddington accretion and one of the fastest superluminal radio jets in the Milky Way. In this presentation, I will share preliminary results from the study of these three microquasars in HAWC's data, including V4641 Sgr, and explore potential mechanisms for gamma-ray production. Additionally, I will outline future efforts to identify more X-ray binaries using HAWC.

Ava Webber (Clemson University)

A Systematic Study of Galactic Star-Forming Regions

Star-forming regions (SFRs) are known sites of particle acceleration and may potentially be important sources of gamma-rays in the Galaxy. While a few Galactic SFRs have been associated with gamma-ray sources, the majority lack detections. Using all available Fermi data in combination with improved optical measurements, we are performing a systematic study of Galactic SFRs. The goal of our systematic study is to characterize the morphology and spectral emission of Galactic SFRs, and understand their energetics and particle acceleration processes. In this talk I will discuss the results of this project focusing on the connection of SFRs' emission and their physical properties.

Nicholas White (George Washington University); T-C. Chang (JPL), D.A. Kann (Goethe), A.J. Levan (Radboud) and A. Lidz (UPenn)

Using High Redshift GRB to Directly Measure Reionization and Early Metal Enrichment

Gamma Ray Bursts (GRBs) were proposed over 20 yr ago as Cosmological probes. They have a featureless power law continuum that can be used as a backlight to directly measure Lyman-alpha and metal absorption lines to constrain the ionization state and metallicity of the intervening intergalactic medium (IGM) and host galaxy interstellar medium (ISM). Fisher matrix analysis is used to forecast the expected constraints on the reionization history using NIR spectroscopy (R> 2,500) from large > 6m class telescopes from a sample of z > 6 GRB afterglows. This spectroscopy simultaneously provides metallicity determinations in low-mass galaxies that are generally too faint for emission line observations, revealing abundance patterns inaccessible by other means. Future missions are being proposed optimized to enable these high redshift measurements including THESEUS (ESA), HiZ-Gundam (JAXA) and Gamow (NASA). We use historical afterglow lightcurves to estimate the follow-up response times required to obtain the required NIR spectra of the high redshift events with the current 6-10m class ground facilities, JWST and the planned 25-40m extremely large telescopes. Follow-up strategies to make these measurements will be discussed. We find a modest aperture NIR photo-z telescope onboard a rapidly slewing spacecraft is essential to identify the few percent of high redshift GRB events and minimize the delay in triggering the follow-up measurements.

Jooyun Woo (Columbia University); The VERITAS collaboration

Multi-epoch spectroscopy of Cassiopeia A using the 14-year Fermi-LAT and VERITAS data

The recent NuSTAR observation of Cassiopeia A (Cas A) discovered the fading of this young Galactic supernova remnant in the hard X-ray band. This fading indicates rapid synchrotron cooling of the most energetic electrons in a highly amplified magnetic field, as well as possible ongoing electron acceleration. While the cooling of electrons will lead to a decrease in leptonic gamma-ray flux, protons freshly accelerated along with electrons will increase the hadronic gamma-ray flux. Time-resolved gamma-ray spectroscopy of Cas A should allow us to disentangle the leptonic and hadronic gamma rays, and hence, estimate the contribution of Cas A to Galactic cosmic ray protons. We plan to present the multi-epoch Fermi-LAT and VERITAS analysis of Cas A using the data taken over the past 14 years. We model the multi-wavelength spectral energy distribution of Cas A by combining the gamma-ray data with the X-ray and radio data. We propose to discuss the implications of these results for the recent theory of modified nonlinear diffusive shock acceleration, and the potential of young SNRs as Galactic PeVatrons.

Sohyoun Yun-Cárcamo (University of Maryland); Dezhi Huang, Rishi Babu, Jason Fan; HAWC Collaboration

Observation of Ultra-High-Energy Gamma Rays from the Galactic Center PeVatron with the HAWC Observatory

Gamma rays with energies greater than 100 TeV have been detected from the Galactic center region using data collected by the HAWC observatory over seven years. The data from this region is best described by a point source model (HAWC J1746-2856) with a simple power-law spectrum. This result extends previous observations of the region to extremely high energies with no cutoff, indicating the presence of a PeVatron at the Galactic Center. The upcoming talk will delve deeper into the modeling of the region and explore the potential origins of the PeVatron.

Ao Zhang (Washington University in St. Louis); Wenlei Chen, Manel Errando

Search for anisotropic pair halos associated with blazar jets

Magnetic fields found in the galaxies are believed to grow from weaker "seed" fields whose origin is still unknown. These weaker "seed" fields could be preserved after the Universe's recombination and stored within large voids filled with the intergalactic medium in the form of a non-zero intergalactic magnetic field (IGMF). Traditional methods such as the Faraday rotation and Zeeman effect have failed to detect a significant level of IGMF, we have found the optimal redshift range at which pair halos show the highest detectability considering the halo brightness and its angular extent compared to the Fermi-LAT angular resolution. We will use these results to identify high-synchrotron-peaked BL Lac objects, whose jet directions are known from radio interferometry measurements, and combine the Fermi-LAT data on their fields of view to search for signatures of anisotropic pair halos. We will also discuss a novel search for pair halos produced by IGMFs that have coherence length larger than 100Mpc. The sensitivity of our search is improved by the directional nature of the pair halo signal and could lead to a potentially significant detection of pair halos for the first time, or tighter constraints to the intensity of the IGMF compared to existing searches.

Jin-Ping Zhu (Monash University)

Formation of GW230529 from Isolated Binary Evolution and Its Electromagnetic Counterparts

We explore the formation of the mass-gap black hole-neutron star (mgBHNS) merger detected in gravitational wave (GW) event, i.e., GW230529, from the isolated binary evolution channel, and study potential signatures of its electromagnetic counterparts. By adopting the `delayed' supernova prescription and reasonable model realizations, our population synthesis simulation results can simultaneously match the rate densities of mgBHNS and total BHNS mergers inferred by the population analyses, along with the population distribution of the BH mass in BHNS mergers reported by the LIGO-Virgo-KAGRA Collaboration. Because GW230529 contributes significantly to the inferred mgBHNS rate densities, we suggest that GW230529 can be explained through the isolated binary evolution channel. Considering the equation of states of AP4 and DD2, the probabilities that GW230529 can make tidal disruption are 12.8% and 63.2%. If GW230529 is a disrupted event, the associated kilonova is predicted to have an apparent magnitude of ~23-24, and hence, can be detected by the present survey projects and LSST. Since GW230529 could be an off-axis event inferred from the GW observation, its gamma-ray burst (GRB) might be too dim to be observed by γ-ray detectors, interpreting the lack of GRB observations. The detection of GW230529 confirms the existence of mgBHNS mergers formed through the isolated binary evolution channel, suggesting that BHNS mergers are still likely to be multimessenger sources that emit GWs, GRBs, and kilonovae. Although mgBHNS mergers account for ~60% cosmological BHNS population, we find that ≳90% disrupted BHNS mergers are expected to originate from mgBHNS mergers.

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11th International Fermi Symposium - Talk Abstracts

11^th International Fermi Symposium - Talk Abstracts