11^th International Fermi Symposium - Talk Abstracts

Sagar Adhikari (Clemson University); P. Penil, J.R. Westernacher-Schneider, A. Dominguez, M. Ajello , S. Buson, A. Rico, J. Zrake

Interpreting hints of a new, 22-year period in PG 1553+113 LC

PG 1553+113 is a blazar, which exhibits a 2.2-year periodic emission detected from radio to gamma rays. Here, we present the first evidence of a 22-year periodic emission detected in the >100-year optical light curve provided by the DASCH database. This is interpreted due to an overdensity of material in the circumbinary disk, which orbits around the black-hole binary on a period approximately ten times longer than the binary itself. This new period supports the argument that a binary supermassive black hole resides at the core of PG 1553+113. This talk will also discuss what can be learned about the binary from these detections.

Tamador Aldowma (University of Johannesburg); Seobur Razzaque, Francesco Longo, Riccardo Martinelli

Probing Gamma-Ray Bursts as Cosmological Standard Candles using Spectral information in Machine Learning models

Gamma-ray bursts (GRBs) represent the most potent explosions detected across electromagnetic wavebands, originating from the death of massive stars (long-duration GRBs) or the merger of two neutron stars (short-duration GRBs). GRBs have been observed from redshift up to z = 9.2, underlining their significance as cosmic probes similar to supernovae type Ia. Extensive efforts have been devoted to using GRBs as cosmological standard candles, leading to the discovery of several empirical relations. This study uses GRB data from the Fermi-GBM and Kouns-Wind catalogs to estimate pseudo-redshifts for several GRBs. This estimation is accomplished by applying Machine Learning techniques, specifically deep neural networks in stacking with the Random Forest algorithm, forming an ensemble model. Subsequently, we conduct a joint spectral analysis using Fermi (GBM and LAT) and Swift (BAT and XRT) data to attain the most optimal fit using ThreeML software. Using the best-fit spectral parameters alongside the pseudo-redshift data, we investigate the Amati relation, a correlation between the intrinsic peak energy (Ei, peak) of the spectrum and isotropic energy (Eiso) over the burst duration (T90). Furthermore, we also explore the Yonetoku relation, a correlation between Ei, peak, and the peak isotropic luminosity (Liso). These analyses aim to deepen our understanding of GRB characteristics and their potential as cosmological standard candles.

Robin Anthony-Petersen (University of California, Berkeley); John Tomsick, Andreas Zoglauer; Compton Spectrometer and Imager (COSI)

Quantifying detector stack alignment for the Compton Spectrometer and Imager (COSI)

The Compton Spectrometer and Imager (COSI), a NASA Small Explorer (SMEX) satellite mission expected to launch in 2027, will utilize the multiple scatters of a gamma-ray's within its detector volume to image the soft gamma-ray sky with unmatched energy and angular resolution. To accomplish this, COSI will consist of sixteen 8×8×1.5 semiconductor germanium detectors (GeDs) arranged in a 4×4 array. To localize a gamma-ray's source, COSI calculates the initial Compton scattering angle by reconstructing the path a fully absorbed gamma-ray takes through its detector volume. The accuracy of this imaging is entirely dependent upon COSI's ability to correctly reconstruct the gamma-ray's path, a task that requires knowing the precise location that each interaction took place within the detector volume. This makes it essential to have accurate alignment of each detector in the 4×4 array as any uncalibrated misalignments within the detector array will lead to error. Here we use simulations to determine the effects of misalignments on the accuracy of COSI's measurements and quantify its tolerance for misalignment. We also investigate possible ways to measure misalignments that could occur during and post launch, including the possibility of using muon tracks to measure the alignment.

Robin Anthony-Petersen (University of California, Berkeley); John Tomsick, Andreas Zoglauer; Compton Spectrometer and Imager (COSI)

Enhancing the Compton Spectrometer and Imager's event reconstruction capabilities with Machine Learning

Compton telescopes are on the forefront of efforts to image the historically under-observed soft gamma-ray sky. They utilize a gamma-ray Compton scattering one or more times in their detector volume to reconstruct the photon's path and localize its origin. While seemingly simple, for compact Compton telescopes the task of reconstructing the correct sequence of scatters is extremely challenging. This is due to the fact that the time between successive interactions is shorter than the readout timing of the detector, and as a result, the correct path of the gamma-ray is not explicit in the raw data and must instead be extracted in the analysis by determining the most probable sequence given the physics of the interaction - a computationally demanding task. To improve current event reconstruction techniques, we propose integrating Machine Learning algorithms into this analysis step. Here we consider the implications of Machine Learning for the Compton Spectrometer and Imager (COSI), a NASA Small Explorer (SMEX) satellite mission expected to launch in 2027. We test four different approaches to Compton event reconstruction for COSI: the classic Compton sequence reconstruction, a naive Bayesian, a random forest, and a neural network approach. These approaches were implemented in the Medium-Energy Gamma-ray Astronomy library (MEGAlib) software toolkit, which is also where we simulated the data, trained the approaches, and evaluated the results. Our findings show that these new methods outperform the classic event reconstruction approach and result in the lowest rates for wrongly reconstructed event paths.

Alessandra Azzollini (Julius-Maximilians-Universität Würzburg, Fakultät für Physik und Astronomie, Emil-Fischer-Str. 31, D-97074 Würzburg, Germany); Sara Buson, Alexis Coleiro, Gaëtan Fichet de Clairfontaine, Leonard Pfeiffer, Massimiliano Lincetto

The physical properties of candidate neutrino-emitter blazars

High-energy neutrinos detected by the IceCube Observatory provide an exclusive opportunity to study the origin of cosmic rays and the nature of the sources producing them. Among the putative birthplaces of astrophysical high-energy neutrinos, blazar jets stand out due to their capability of accelerating particles and providing intense external radiation fields. Blazars are Active Galactic Nuclei (AGN), a class of luminous extra-galactic objects that are powered by a central supermassive black hole, with the jets pointing in the observer's line of sight.
In this contribution, we focus on a small subsample of blazars that have been put forward as candidate IceCube neutrino counterparts. We characterize the sources' nature and their central engine's peculiarities. We use multi-wavelength data, both archival and proprietary, in the radio, optical, and γ-ray bands and provide new insights into the intrinsic properties of this subpopulation of neutrino-emitter candidates. Properties such as redshift, black hole mass, accretion regime, radiation field, and jet power are crucial clues to investigate the properties of these blazars and the potential link with the acceleration of cosmic rays.
We will present our findings, and place our results in the general context of the properties displayed by the γ-ray blazar population.

Priyadarshini Bangale (University of Delaware); Xiaojie Wang; VERITAS and HAWC

Search for TeV emission from a recently Fermi-LAT detected SNR G17.8+16.7 and a bright unidentified PeVatron LHAASO J0341+5258

The success of the Fermi-LAT satellite, the current generation of Cherenkov telescopes, and particle detector arrays has led to exciting discoveries of gamma-ray sources. These discoveries include new hard spectrum supernova remnants (SNRs) and ultra-high-energy Galactic PeVatron sources accelerating particles to a few PeV (~10¹⁵ eV) energies. Here, we present results from two contrasting extended Galactic sources: Fermi-LAT/VERITAS follow-up of recently Fermi-LAT detected SNR G17.8+16.7 and Fermi-LAT/VERITAS/HAWC follow-up of LHAASO detected PeVatron J0341+5258. Both sources are distinctive in nature and provide insights into the non-thermal Universe from lowest to highest energies in the gamma-ray regime. SNR G17.8+16.7 exhibits a shell-like morphology with an angular size of 0.8 deg. It is bright above 100 GeV and exhibits a hard power-law spectrum, making it a good candidate for follow-up studies for the TeV energies. LHAASO J0341+5258 has been detected as an unidentified PeVatron, emitting gamma rays at energies above hundreds of TeV. It is extended in nature and notably bright, with a flux > 20% of the Crab Nebula's flux above 25 TeV. Multiwavelength observations are required to identify the objects responsible for the ultra-high-energy gamma rays, to understand the source morphology and association, and to shed light on the emission processes. Here, we present the search for the VHE emission and broadband modeling results to study further the properties of these SNR and PeVatron sources and their surrounding environments, which will help us establish the nature of these sources and their emission mechanisms.

Daniel Barr (Michigan Technological University); Pfesesani Van Zyl, Tiffany Lewis, Jeffrey Hodgson; Fermi-LAT Collaboration

Exploring the radio emission connection to the blazar zone: The case of PKS 1424-418

We investigate the potential correlated relationships in multi-wavelength light curves to establish whether multi-resolution radio data can be used to support models claiming that the higher energy gamma-ray emission leads the radio emission. We have been continuously monitoring the flat spectrum radio quasar PKS 1424-418 in the S, C, X, and Ku radio bands throughout the duration of the Fermi mission. We use discrete correlation functions to estimate the correlations and time lags between the radio and gamma-ray emission. Alongside the time series analysis, we apply a theoretical model to simultaneous broad-band spectral energy distributions from radio to gamma-rays to explore possible interpretations of correlated and uncorrelated lags.

Deven Bhakta (University of Virginia/NRAO); Scott Ransom, Matthew Kerr, Paul Ray

Proper Motion Measurements of Radio-Quiet Pulsars Using γ-ray Single photon Timing

Roughly 300 γ-ray pulsars have been detected across the 16 years of Fermi's continuous observing. Nearly half of these pulsars are young pulsars. Measuring the proper motion of these young pulsars informs us about the birth mechanisms and velocities/kicks of neutron stars, and is possible via pulsar timing. γ-ray pulsar timing is generally more difficult than radio timing due to the low number of collected γ-ray photons and high confusion with background sources. A good signal-to-noise ratio radio pulse time-of-arrival requires averaging over 10^2 to 10^3 pulses for slow pulsars and can be achieved within an hour of radio observations. The similar γ-ray analysis would require averaging weeks or months of data due to the extremely low photon count rate. Thus, it is beneficial to try and extract the most information from each detected photon. We are using Bayesian analysis and MCMC techniques within PINT to time radio-quiet young pulsars with single photons. Despite timing individual photons, we can take advantage of the long, 16-yr Fermi-LAT data set to measure properties like proper motion! Several of our pulsars show significant red noise, which is correlated noise that has higher power at lower frequencies, and causes the pulse phase to wander. We are using a more complex noise analysis that uses summed sinusoidal waves alongside our Bayesian MCMC analysis to account for this noise. I will present our source selection criteria, the techniques we used to measure the proper motions of young radio-quiet γ-ray pulsars, and our results.

Krishiv Bhatia (Mountain View High School); Pablo Saz Parkinson; Fermi LAT Collaboration

The application of machine learning techniques to the problem of source classification with Fermi-LAT

Since the beginning of the Fermi mission, in 2008, the LAT Collaboration has released a large number of catalogs; from the 3-month Bright Source List (0FGL) to the recent 14-year 4th Data Release of the 4th Fermi LAT Source Catalog (4FGL DR4). While the number of sources and source classes continues to grow, and our knowledge of the gamma-ray sky continues to improve, the fraction of unassociated sources remains sizeable (~ 1/3). I will present an overview of our understanding of the various gamma-ray source classes over the years and how machine learning techniques are helping to discriminate between them. I will also discuss some of the challenges faced by these techniques and the prospects for uncovering the nature of the remaining unassociated sources.

Markus Boettcher (North-West University); Ogochukwu Chibueze; H.E.S.S. Collaboration

H.E.S.S. detection and multiwavelength study of the z ~ 1 blazar PKS 0346-27

We report the detection of a TeV blazar PKS 0346-27 at redshift 0.99 by the High Energy Stereoscopic System (H.E.S.S.) on 3rd November, 2021 with a significance above 5 σ. The spectral energy distribution (SED) consists of the simultaneous observations by Fermi-LAT, Swift XRT and UVOT during the H.E.S.S detection period. We show that a hadronic onezone model (modified by strong EBL absorption) can provide a satisfactory fit to the data. The lightcurve consists of the multiwavelength data for all the observation periods and we were able to test some time lag between the GeV and TeV bands.

Seth Bruzewski (United States Naval Observatory); Frank Schinzel

Radio Updates to Fermi Gamma-Ray Sources

In the time since the launch of the Fermi Gamma-Ray Space Telescope, observations from the radio regime have played a significant role in finding associations and identifications for the various astrophysics sources which are seen by Fermi in the gamma-ray regime. Efforts to provide further counterparts from the radio regime are ongoing, and here we will provide updates on two particular projects of note: a Very Long Baseline Array (VLBA) search for new AGN associations involving new target selection criteria, and the refinement of the proper motion for the identified gamma-ray pulsar J0002+6216 (the ""Cannonball Pulsar"").

Robert Cameron (KIPAC/SLAC, Stanford); Elizabeth Hays, Tyrel Johnson; Fermi LAT Collaboration

LAT Performance After 16 Years in Orbit

The Fermi observatory, and the Large Area Telescope (LAT) on Fermi, were designed and built with an operating lifetime goal of 10 years. The Fermi LAT now has been successfully operating in low Earth orbit almost continuously since its initial turn-on on 24 June 2008, for over 16 years. Details are presented of the current performance of the LAT detector and data acquisition sub-systems together with long-term trends of key performance measures and we assess their expected future performance. LAT ground-based operations and data processing are jointly supported by NASA, SLAC, and the Fermi LAT Collaboration.

Paolo Coppi (Yale University); Andrea Bulgarelli, Marco Tavani

Short-Timescale Variability in Blazars as Seen by AGILE and FERMI

While many gamma-ray blazar variability studies rely on lightcurves binned on daily or weekly intervals, we now know that blazars, even FSRQs, can vary on timescales as short as minutes. To better understand the implications of this, we have developed a fast aperture photometry pipeline for bright sources that can analyze AGILE and FERMI data on sub-hour timescales and enables a joint fit when the coverage of the two missions overlaps. Joint fitting by detectors with different systematics improves the significance and believability of otherwise marginal candidate flares. When the coverage of the two satellites is not strictly simultaneous and a joint fit is not possible, the different coverage patterns of the satellites nonetheless result in better overall time-sampling of lightcurves. We present preliminary results from our use of the pipeline, including hints that optical and gamma-ray variability do not track each other on the shortest timescales, which is problematic for models.

Robin Corbet (UMBC/NASA GSFC/CRESST/MICA); L. Chomiuk, H. Cohen, G. Dubus, P. Edwards, N. Islam, Alex Lange, J. Stevens, J. Strader, L. Townsend

A Search for Short Orbital Period Binaries (and Long-Period Pulsars)

A variety of gamma-ray sources can produce emission that is periodically modulated. At the highest frequencies/shortest periods this is produced by rapidly rotating neutron stars, in particular ms pulsars for the very shortest coherent periods. At longer periods, binary sources can have their modulation modulated on the orbital period of the system due to either the changing viewing angle, or due to the eccentricity of the system. We are undertaking a search for binary systems using high-end computing to expand the search to much higher frequencies/shorter periods than has previously been explored for this type of search. This has the potential to both detect ""exotic"" types of systems, and also long-period pulsars. Such binaries might include short orbital period spider systems, ultra-compact X-ray binaries, or Double Compact-Object Systems. Our current search is exploring periods down to 0.005 days (432 s). Candidate periodicities are followed up using a variety of multi-wavelength observations to both confirm the periodicities, and search for counterparts. We will present current results and our plans to extend this to even shorter periods.

Paolo Da Vela (OAS INAF Bologna); Lara Nava, Davide Miceli, Giancarlo Ghirlanda

Prospect for detection of pair-echo emission from TeV gamma-ray bursts

The presence of pair echo GeV emission after a Gamma Ray Burst (GRB) detected in the very-high energy band (VHE, E>100 GeV) can be the signature of the existence of 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. In presence of a not negligible Intergalactic Magnetic Field (IGMF) this emission is delayed. In this contribution we propose a study of the evolution of the pair echo emission during the afterglow of the GRB. We use simulations to estimate the pair echo lightcurves induced by the propagation of primary VHE gamma rays injected instantaneously by the source. The expected pair echo lightcurve is computed convolving the variability pattern of the GRB in the VHE band with the simulation output. We followed this procedure simulating the pair echo signal from a selected synthetic population of GRBs and producing the pair echo lightcurves in the GeV domain for different IGMF strengths. We show that, depending on the characteristics of the GRBs (e.g. jet opening angle, distance and energetic), the pair echo signal may be dominant for late times with respect to the afterglow.

Sarah Dalessi (Department of Space Science, The University of Alabama in Huntsville; Center for Space Plasma and Aeronomic Research (CSPAR), The University of Alabama in Huntsville); Katherine Davidson, Rebecca Harvey, Laura Provenzani; Future Technologies and Enabling Plasma Processes (FTPP), NSF Established Program to Stimulate Competitive Research (EPSCoR)

DEI and Outreach efforts from the NSF FTPP consortium

The NSF Established Program to Stimulate Competitive Research (EPSCoR) Future Technologies and Enabling Plasma Processes (FTPP) is a 10-member university and industry consortium in Alabama. The project leverages Alabama's laboratory and space plasma science expertise to contribute scientific knowledge, technologies, and workforce to Alabama's aerospace, manufacturing, advanced materials, medical, bioscience, and agricultural sectors. FTPP aims to promote diversity, equity, and inclusion within Alabama's research, education, and workforce development. FTPP's diversity efforts will be highlighted, including the strategic sharing of resources between Alabama's R1 universities and HBCUs. FTPP provides internship opportunities for HBCUs, partnerships with Alpha Kappa Alpha Power of Internships in South Eastern (POISE) for internship placements, and increasing awareness of the various projects and diversity efforts across all EPSCoR states. In concurrence with DEI efforts, FTPP aims to increase awareness of plasma physics and related projects by participating in several outreach programs. Due to these efforts, FTPP was able to increase its internship application pool by 62% from 2023 to 2024. The accepted applicants' demographic information is similar to or more diverse than the national average demographics for STEM fields. Through FTPP outreach efforts, plasma physics and STEM concepts were introduced to 2000+ people in Alabama through outreach efforts such as an elementary STEM day workshop, eclipse activity events for the October 14, 2023 and April 8, 2024 eclipses. Future workshops are planned in association with the American Association of University Women Tech Trek program, the Rocket City Astronomy on Tap lecture program, and the Arkansas Regional Innovation Hub.

Margherita De Toma (SISSA); Buson S., Larsson S., Cheung C., Ciprini S., P. Cristarella Orestano on behalf of the Fermi-LAT collaboration; Fermi-LAT collaboration

Measurement of the time delay in the gravitationally lensed system PKS 1830-211

The gravitationally lensed blazar PKS 1830-211 underwent a historically bright and unusually long-duration gamma-ray flaring episode in 2019/2021 with daily fluxes exceeding >1e-6 ph/cm2/s for ~400 days, and peak fluxes (>1e-5 ph/cm2/s) exceeding all prior flares observed in the 15 years of the full Fermi-LAT mission dataset. We analyzed six bright flaring episodes (~hundreds of days long) independently using an auto-correlation function analysis, improved by a new methodology for detrending of the gamma-ray light curve, i.e. the removal of the underlying low-frequency trend due to the red noise stochastic components. We identify a statistically significant (>5sigma) delay in the gamma-ray data of the order of 20 days. The value is consistent over the different observational epochs, and is attributable to the gravitational lensing effect. The results and uncertainties presented here account for stochastic variability of the blazar for the first time and, in contrast to prior works that considered the earliest fainter flaring intervals and reported disparate time delay values in gamma rays, we here consider improvements in the reliability of the LAT lightcurves, and benefits from improvements enabled by the present LAT calibrations. In this contribution, we present these new findings, and discuss how the possible discrepancy between the time delay measured at gamma-rays (20 days) and the one derived from radio observations (~25 days) may be reconciled, e.g. as a probe of different emission regions responsible for the gamma rays and radio emission, and/or microlensing effects.

Julia Deneva (George Mason University, resident at the Naval Research Laboratory); Paul Ray, Mallory Roberts, Scott Ransom, Thankful Cromartie, Megan DeCesar, Shawaiz Tabassum

Timing of Pulsars Discovered in Jerk Searches of Fermi Unassociated Sources

The Fermi Pulsar Search Consortium (PSC) has discovered 139 pulsars in Fermi unassociated sources using the world's most sensitive radio telescopes. We present a status update on the timing of ten millisecond pulsars found with the Green Bank telescope. In order to fully characterize these discoveries, we perform regular radio observations over at least one year with the goal of obtaining phase-coherent rotational ephemerides. These are then used as a starting point to search for gamma-ray pulsations in Fermi LAT data by folding the sparse photons originating from the direction to the pulsar. If gamma-ray pulsations are detected, the rotational ephemerides can be extended to the full 16-year span of the Fermi mission. We aim to determine if the new MSPs are associated with the Fermi sources, contribute to future Fermi Pulsar Catalogs, magnetospheric geometry studies and multi-wavelength pulsar emission models, and evaluate the MSPs for inclusion in pulsar timing arrays for the detection of gravitational waves.

Qi Feng (University of Utah); The CTAO SCT Project

Science Perspectives of the Cherenkov Telescope Array Observatory

The Cherenkov Telescope Array Observatory (CTAO) is the next-generation ground-based gamma-ray observatory, aiming to achieve superior sensitivity and angular resolution over a broader energy range than current instruments. The CTAO includes two extensive arrays of atmospheric Cherenkov telescopes of three different sizes, located in both the southern and northern hemispheres. One of the medium-sized telescope designs that may be used for the CTAO is the dual-mirror Schwarzschild-Couder Telescope (SCT), offering better optical imaging capability across the field of view compared to its single-mirror counterpart. The CTAO supports a diverse scientific program that covers three main areas: cosmic-ray particle acceleration, astrophysics within extreme environments, and physics beyond the Standard Model. This presentation outlines the scientific objectives of the CTAO and showcases the improved angular resolution and sensitivity that the dual-mirror SCTs could bring to the CTAO.

Justin Finke (US Naval Research Laboratory); Lee Mitchell, Neil Johnson, Emily Kong

SIRI-2 Detection of the Gamma-ray Burst 221009A

SIRI-2 is a collection of Strontium Iodide gamma-ray detectors sensitive at approximately 400 keV to 10 MeV, launched on the Department of Defense's STPSat-6 to geosynchronous orbit. SIRI-2 detected the gamma-ray burst (GRB) 221009A and, unlike most GRB detectors, was not saturated and did not require any pulse pile-up corrections. We present observations of this GRB by SIRI-2, including spectral fits and comparison with other detectors.

Justin Finke (US Naval Research Laboratory); Soebur Razzaque

Is Gamma-ray Burst 221009A Really a Once-in-10,000 Year Event?

Extrapolation of the gamma-ray burst fluence distribution to the fluence of gamma-ray burst (GRB) 221009A, the brightest ever detected, leads to the conclusion that GRBs brighter than this burst should occur approximately once per 10,000 years. It would be a large coincidence if such a GRB occurred in the approximately 50 years that humanity has had the ability to detect such bursts. Here we propose that GRB 221009A is part of a separate, nearby population of narrow-jet GRBs. This population can allow GRBs as bright or brighter than 221009A to occur as often as once every 200 years without over-producing the observed rate of other GRBs. We explore observational implications of this model.

Yasushi Fukazawa (Hiroshima University); Hiroto Matake

Jet power of gamma-ray emitting radio galaxies and relation with accretion power

Radio galaxies has been established as gamma-ray sources by Fermi satellite. So ar more than 60 radio galaxies have been detected by Fermi/LAT. Gamma-ray emission of radio galaxies is thought to be the jet emission and thus we can derive jet power by analysis of SED. Therefore, we have systematically analyzed gamma-ray emitting radio galaxies. SED of radio galaxies are not easy to understand, because other emission components than jet are also observed in the X-ray band. First, in order to constrain SED, we studied whether X-ray is dominated by jet or disk/corona emission. Then, we consider this property to fit the SED from radio to GeV gamma-ray by the SSC model. Using obtained SSC parameters, we derived jet power. We also derived jet power from radio flux referring to the past studies. We also derived the jet powers for blazers, It is found that the jet power of gamma-ray emitting radio galaxies is similar to that of blazar when we use the jet power derived from SED fitting. On the other hand, jet power derived from radio flux distributes widely for blazars or smaller by two orders of magnitude for radio galaxies. This indicates the jet power derived from radio flux should be paid attention by considering jet viewing angle. As a result, jet power of gamma-ray emitting radio galaxies becomes larger by a factor of 10 than accretion power as well as reported for blazars. This indicates a BH rotation energy for jet formation.

Seth Gagnon (The George Washington University)l Noel Klinger, Oleg Kargaltsev

CXO Analysis of PSR J1849-0001 and its Pulsar Wind Nebula

The Chandra X-ray Observatory (CXO) observed PSR J1849-0001 and its PWN, coincident with the TeV source HESS J1849-000, for 108-ks. Our analysis of the new and old (archival) CXO data allowed for resolution of the pulsar from the PWN, exploration of the PWN morphology on arcsecond and arcminute scales, and spectral analysis of different regions of the PWN. Both the pulsar and the compact inner PWN spectra are hard with power-law photon indices of 1.20 ± 0.07 and 1.49 ± 0.20 respectively. The jet-dominated PWN with a relatively low luminosity, lack of γ-ray pulsations, relatively hard and non-thermal spectrum of the pulsar, and its sine-like pulse profile, indicate a relatively small angle between the pulsar's spin and magnetic dipole axis. In this respect, it shares similar properties with a few other so-called MeV pulsars. Single-zone modeling can roughly describe the joint X-ray and TeV SED, although the obtained magnetic field value is unrealistically low. A more realistic scenario is the presence of a relic PWN, no longer emitting synchrotron X-rays but still radiating in TeV via inverse-Compton upscattering. We also provide a comparison to a similar source, CTA 1, which also lacks clear evidence for a torus, but has an obvious jet structure and pulsations have been measured up to gamma-ray energies.

Savitri Gallego (Johannes Gutenberg University Mainz); COSI

Background model and simulation for the Compton spectrometer and Imager

The Compton Spectrometer and Imager (COSI) is a gamma-ray telescope in low-Earth orbit selected by NASA as a Small Explorer satellite mission to be launched in 2027. COSI employs a novel Compton telescope, consisting of a compact array of cross-strip germanium detectors. Owing to its wide field-of-view and excellent energy resolution, COSI will achieve an unprecedented sensitivity in the 0.2-5 MeV energy band and be highly complementary to the Fermi Gamma Ray Space Telescope.
Gamma-ray satellites are highly dominated by the background. Having a good background model is key for the data analysis. In this talk I will present the full bottom-up simulations of the gamma-ray background done for the new COSI data challenge. In addition, I will present the background simulation done for the 2016 COSI Balloon flight and the comparison with the measurements.

Andrea Gokus (Washington University in Saint Louis); Rebecca Phillipson, Manel Errando

Recurrence plot analysis of blazar gamma-ray light curves: exploiting the time-domain capabilities of Fermi-LAT

Fermi-LAT has accumulated continuous, high signal-to-noise, flux monitoring of bright blazars for over a decade in the gamma-ray band, measuring the dynamics of the particle acceleration and radiation zone in the blazar jet. The statistical methods often used to characterize the measured time variability, such as techniques based on the Fourier transform, rely on the underlying assumption that the time series data is stationary, with mean and variance remaining constant over time. However, this is not the case for blazar light curves.
We will present a different approach to characterize the flux variability observed in bright gamma-ray blazars using the recurrence plot analysis technique.
Recurrence plot analysis is well-suited for non-stationary time series, and can provide a quantitative description of complex as well as recurrent variability patterns, providing insight into the stochastic and chaotic processes that cause the observed flux variability. In addition, recurrence plots are robust against noise fluctuations and are applicable to long data sets such as Fermi-LAT light curves.
We will employ this method to gain insight into the non-linear and stochastic behavior of blazar jets at γ-ray energies and probe the importance of processes occurring in time scales from weeks to several years such as the connection between accretion power and jet launching, quasi-periodic oscillations, dynamics of energized plasma in the blazar jet, and jet precession.

Eric Grove (U.S. Naval Research Laboratory); M. Kerr, R. Woolf, C.C. Cheung, A. Goldstein, C. Wilson-Hodge, D. Kocevski, M. Briggs

Results from the Glowbug Gamma-ray Transient Telescope

Glowbug, a gamma-ray telescope sensitive to gamma rays between 50 keV and 2 MeV, operated on the International Space Station from March 2023 to April 2024, when its planned mission came to an end. Because Glowbug had comparable sensitivity to Fermi GBM but typically looked at a different portion of the sky, it provided key, complementary monitoring and enhanced sky coverage for gamma-ray transients during the first part of the O4 operational run of LIGO/Virgo/KAGRA. We present a brief overview of the instrument and its operation, and we present selected highlights including new gamma-ray bursts detected by Glowbug, and follow-up to gravitational wave event alerts. This work is supported by the NASA Astrophysics Research and Analysis program.

Sylvain Guiriec (George Washington University / NASA Goddard Space Flight Center)

Advancing Our Understanding of GRB Jet Physics and Harnessing their Potential as Cosmological Standard Candles

The presence of both thermal and non-thermal emission components in the early explosion of Gamma-Ray Bursts (GRBs) is well-established. These components have been extensively studied across various wavelengths, from gamma rays to optical, using multiple instruments and space observatories. The spectral characteristics align with the theoretical predictions of the Fireball model, wherein a sub-dominant thermal-like component originates from the jet photospheric emission, while a non-thermal component arises from synchrotron emission through electron acceleration within the jetted outflow, facilitated by internal shocks and/or magnetic reconnections.
In this presentation, we delve into these emission components, illustrating how a physically-motivated model can track their evolution with fine time-resolved spectroscopy, reaching down to the millisecond timescale. By utilizing this model, we can extract vital properties of the relativistic outflow, including the magnetization, Lorentz factor profile, photospheric radius evolution, injected power evolution, and energy injection radius. These outputs from the physically-motivated model subsequently serve as inputs for a purely physical model, which effectively narrows down the extensive parameter space. This reduction enables the fitting of observed data to comprehensive physical model templates, thus constraining the various microphysical parameters that characterize different aspects of the outflow, jet geometry, circumburst medium, internal shocks, and external shocks.
Ultimately, our work establishes GRBs as cosmological standard candles supported by a robust physical framework. These results not only enhance our understanding of GRB jet physics but also contributes to their application as valuable cosmological tools.

Soumya Gupta (Bhabha Atomic Research Center, Mumbai, India / Homi Bhabha National Institute, Mumbai, India); Sunder Sahayanathan, Dipankar Bhattacharya

Investigating GRB 171227A Prompt Phase under the Relativistically Expanding Fireball Scenario

The emission process of the prompt phase of the Gamma-Ray Bursts (GRB) is still an open question. GRB spectrum is often well fitted by an empirical smooth broken power-law function term as the Band function. However, observations from the GLAST Burst Monitor (GBM) onboard the Fermi Gamma-Ray space telescope suggest the presence of a thermal component along with the non-thermal, whose origin is unintelligible. We perform a detailed study of the relativistically expanding fireball model with its temperature evolving as a function of its radius. In this work, we addressed the effect of high optical depth and its dependence on the density profile between the wind and the observer, resulting in the nontrivial shape of the photospheric radius. Further, the effect of the light travel time integrated with the time-averaged spectrum results in a much broader spectrum compared to the Planck function. A numerical code developed under this scenario is employed to reproduce the time-averaged and time-resolved spectrum of one of the bright GRB 171227A. We investigated the details of the prompt phase of GRB 171227A using GBM data. The spectral fitting is performed by coupling this numerical model with the statistical fitting package XSpec.

Jeremy Hare (NASA GSFC/CRESST II/CUA); Hui Yang, Steven Chen, Yichao Lin, Oleg Kargaltsev

Classifying potential X-ray counterparts to unidentified Fermi-LAT sources using automated machine learning methods

The Fermi-LAT 4FGL DR4 catalog contains many unidentified sources. One of the most successful ways to uncover their counterparts is through X-ray observations. Here, we report the results of applying the MUWCLASS machine learning pipeline to X-ray sources located in the fields of 73 unidentified 4FGL sources observed by Chandra and included in version 2.0 of the Chandra source catalog. In total, we classified ~1200 X-ray sources located within the positional uncertainties of the 4FGL sources. This study uncovered 87 plausible X-ray counterparts to 33 4FGL sources, including 2 isolated neutron star candidates, 16 AGN candidates, 7 sources associated with star-forming regions, and 8 ambiguous cases. In the remaining 40 unidentified 4FGL sources, either no plausible X-ray counterpart was identified, or the 4FGL source was located in a crowded environment (e.g., near the Galactic center), making classification difficult. Lastly, we will discuss our efforts to expand the pipeline to use XMM-Newton and eROSITA X-ray observations, which will allow us to search for counterparts to additional unidentified 4FGL sources, and the challenges that arise.

Dezhi Huang (University of Maryland, College Park); HAWC

Probing Cosmic-Ray Acceleration in the W51 Complex with HAWC Observatory

Located half a degree below the galactic plane, the W51 complex is known for its diverse gamma-ray sources, including supernova remnant (SNR), pulsar wind nebula (PWN), and start forming region (SFR), continues to be a significant region for understanding cosmic-ray acceleration. Recent observations from LHAASO have extended the gamma-ray energy spectrum up to 200 TeV, favoring a hadronic process and reinforcing the hypothesis that SNRs can accelerate particles to energies exceeding the PeV scale. In this presentation, I will analyze the gamma-ray emissions from W51 as observed by HAWC, utilizing the newly improved Pass 5 reconstruction data. The discussion will focus on their spectral and spatial modeling and the potential association of these emissions with cosmic-ray accelerators within the region.

Constantinos Kalapotharakos (NASA, Goddard Space Flight Center); Z. Wadiasingh, A. Harding, D. Kazanas

Beyond the Fundamental Plane: Exploring the Limits and Potentials of Gamma-Ray Pulsars

We introduce particle-in-cell models that not only align with the fundamental plane of gamma-ray pulsars and observed gamma-ray light-curve patterns but also challenge conventional estimates of total gamma-ray luminosities in Fermi catalogs. Our investigation sheds light on why Fermi pulsars occupy specific regions within the fundamental plane. We reveal the critical role of the radiation reaction limit, defining the 'death lines' and 'death valleys' for young and millisecond gamma-ray pulsars. This exploration suggests the existence of an undetected population of MeV pulsars, highlighting the potential of upcoming MeV space telescope missions like AMEGO-X. Our findings also align with recent discoveries, such as the detection of very-high-energy (up to 20 TeV) emission from the Vela pulsar. Moreover, we will discuss the placement of magnetars within the fundamental plane, reasons for their non-detection with LAT, and the energies at which they might emit pulsed emission based on our model considerations and existing observations.

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.

Oleg Kargaltsev (The George Washington University); Yichao Lin, Jeremy Hare, Hui Yang

Multiwavelength View of LHAASO Pevatrons

We present a compilation of multiwavelength properties for ~40 sources detected by LHAASO above 100 TeV. The multiwavelength data include new-generation radio surveys, infrared surveys, X-ray imaging observations, and gamma-ray data <100 TeV. Using our compilation we analyze the environments of LHAASO sources to shed light on the nature of particle accelerators powering these UHE sources. We also investigate the nature of promising X-ray sources within the extent of LHAASO sources and identify optimal future observing strategies.

Chris Karwin (NASA Goddard Space Flight Center); on behalf of the GALE team; GALE

Study of the fine structure of the 511 keV Galactic positron annihilation with the GALE balloon flight

We present a project recently proposed to the NASA APRA program that addresses the long-standing problem of the nature of Galactic positrons: whether they are produced in unresolved faint point sources, or diffusely throughout the Galactic Center (GC), possibly of an "exotic" nature. This problem cannot be fully resolved by any currently operating (INTEGRAL) or planned (COSI) gamma-ray missions, since neither of them combines the requisite angular resolution and point-source sensitivity needed to understand the annihilation emission structure.
The required point-source sensitivity and angular resolution can be achieved in an ultra-long duration balloon flight (ULDBF) of a high-sensitivity, coded-aperture telescope. At the heart of the instrument, the Galactic Annihilation Line Explorer (GALE), is the cadmium zinc telluride gamma-ray Imaging Calorimeter. To achieve the pointing accuracy required by the measurements, GALE will be mated to the Wallops Arc-Second Pointer (WASP).
The project is planned for execution in two steps: first is a 1-day engineering flight from Ft. Sumner, NM, to test the GALE and WASP joint operation, followed by a science ULDBF in the southern hemisphere. GALE will have an angular resolution <20', an energy resolution of 1-2% at 511 keV, a 5 degree fully coded field of view, and a narrow-line 5-sigma point-source sensitivity of 7 x 10-5 ph/cm2/s, assuming a 46-day flight. In such a flight we will be able to determine if the central 511 keV source in the GC is truly extended, having a FWHM > 0.7 degrees, or if it is in fact point-like in nature.

Rukaiya Khatoon (North-West University)

Modeling multiband SED and light curves of BL Lacertae using a time-dependent shock-in-jet model

The origin of fast flux variability in blazars is a long-standing problem, with many theoretical models proposed to explain it. In this study, we focus on BL Lacertae to model its spectral energy distribution (SED) and broadband light curves using a diffusive shock acceleration process involving multiple mildly relativistic shocks, coupled with a time-dependent radiation transfer code. BL Lacertae was the target of a comprehensive multiwavelength monitoring campaign in early July 2021. We present a detailed investigation of the source's broadband spectral and light curve features using simultaneous observations at optical-UV frequencies with Swift-UVOT, in X-rays with Swift-XRT and AstroSat-SXT/LAXPC, and in gamma-rays with Fermi-LAT, covering the period from July to August 2021 (MJD 59400 to 59450). A fractional variability analysis shows that the source is most variable in gamma-rays, followed by X-rays, UV, and optical. This allowed us to determine the fastest variability time in gamma-rays to be on the order of a few hours. The AstroSat-SXT and LAXPC light curves indicate X-ray variability on the order of a few kiloseconds. Modeling simultaneously the SEDs of low and high flux states of the source and the multiband light curves provided insights into the particle acceleration mechanisms at play. This is the first instance of a physical model that accurately captures the multi-band temporal variability of BL Lacertae, including the hour-scale fluctuations observed during the flare

Daewon Kim (Max-Planck-Institute for Radio Astronomy); Eduardo Ros, Matthias Kadler, Thomas P. Krichbaum, Guang-Yao Zhao, Florian Rösch, Andrei P. Lobanov, J. Anton Zensus; AGN / LAT Collaboration

PKS 1424-418: A best case of the blazar radio/gamma-ray connection

Blazars, a subclass of radio-loud AGN are among the best laboratories for high-energy astrophysics in the Universe. The relativistic jets in blazars are prominent gamma-ray emitters with rapid variablity down to minute scales. The underlying physical mechanisms and origin of the gamma-ray emission, however, are not yet fully understood. One of the key diagnoses for the relevant studies is to explore statistically significant correlations between gamma-ray and lower-energy band (e.g., radio-to-optical) light curves in the sources. In this work, we analyzed the correlation with millimeter (> 90GHz) radio light curves in the blazar PKS 1424-418 and found a long-term, tight radio/gamma-ray connection which is atypical compared to the cases of other blazars. The correlation spans ~8.5 years with a small amount of time lag (i.e., less than three days). Given the well-known blazar jet model with the core-shift theory, the results indicate that the gamma-ray production site is spatially connected to the location of the millimeter radio core at e.g., (sub)parsec scales in the jet of PKS 1424-418. Additional analysis of the evolution of radio spectral index (95GHz vs. 345GHz) clearly shows us the coincidence between the spectral hardenings and gamma-ray flares. This further implies that a small displacement between the gamma-ray origin and the radio core may occurs when the source flares at gamma-rays, perhaps due to the passage of a strong moving shock/blob. We suggest that this particular blazar might be a persistent source of the radio/gamma-ray connection.

Daniel Kocevski (NASA); Brian Humensky, Jamie Kennea, Michell Hui, Samuel Wyatt, Christopher Roberts; ACROSS Team

The Astrophysics Cross-Observatory Science Support (ACROSS) Initiative

The Astro2020 Decadal Survey recommended an investment in Time Domain and Multi-Messenger Astrophysics (TDAMM) as the top-priority sustaining activity in space for the coming decade. One aspect of NASA's response to this recommendation is a study, led by the Physics of the Cosmos Program Office, of how a structure akin to a General Observer Facility might be implemented that would provide support to both missions and observers as they pursue TDAMM science. The first year of the study focused on coordination between NASA missions; its chief recommendation was the initiation of the Astrophysics Cross-Observatory Science Support (ACROSS) pilot project, which began in October 2023. This talk covers the status and plans for the ACROSS pilot project, as well as how the gamma-ray community, and Fermi users in particular, can benefit from the tools and infrastructure currently being developed by the ACROSS team.

Daniel Kocevski (NASA); Joshua Wood, Adam Goldstein, Michelle Hui, Colleen Wilson-Hodge; GBM Team

The Gamma-ray Targeted Search

The Fermi GBM Targeted Search has proven to be incredibly efficient at recovering weak signals in untriggered continuous time tagged event (CTTE) data. The GBM team estimates that the search could expand the volume of the universe in which the GBM detects gravitational wave counterparts like GRB 170817A by almost an order of magnitude. Here we introduce a general-purpose rewrite of this analysis method, now available as an open-source tool named the Gamma-ray Targeted Search (GTS). The GTS utilizes the newly available Gamma-ray Data Tools (GDT) and enables users to perform custom searches of GBM CTTE data for sub-threshold signals at specific times. Being built on the GDT, the GTS can also analyze GBM-like data from other missions, such as BurstCube and the upcoming StarBurst mission. We discuss the capabilities of the GTS as well as future plans to enhance its functionality to create a multi-mission targeted search. This expanded search would coherently combine the capabilities of multiple gamma-ray instruments in orbit to conduct ever deeper searches of the gamma-ray sky.

Alex Lange (GWU/NASA GSFC/UMBC); Robin Corbet, Joel Coley, Jeremy Hare, Nazma Islam, Guillaume Dubus, Jonathan Barnes

Orbital phase-resolved analysis of X-ray and Gamma-ray observations of High-Mass Gamma-ray Binary 4FGL J1405.1-6119

We present the results of multi-wavelength observations of the High-Mass Gamma-Ray Binary 4FGL J1405.1-6119. XMM-Newton and NuSTAR observations taken in 2019 constrain the emission of soft and hard X-rays, show variability for nH and spectral index γ and provide no evidence of short-term variability. Additional spectral analysis shows a lack of a thermal component or a cyclotron resonance scattering feature below 2 keV. We also present the first orbital phase-resolved analysis of 14 years of Fermi-LAT data of 4FGL J1405.1-6119 and the evolution of the spectral shape as a function of orbital phase. Finally, the X-ray and γ-ray spectrum can be interpreted and fitted in the frame work of the intrabinary shock model previously used in High-Mass Gamma-Ray binaries such as LS 5039.

Silvia Manconi (LAPTh - CNRS); Jooyun Woo, Ruo-Yu Shang, Roman Krivonos, Claudia Tang, Mattia Di Mauro, Fiorenza Donato, Kaya Mori, Charles J. Hailey

Geminga's pulsar halo: a multiwavelength view

Geminga is the first pulsar around which a remarkable TeV gamma-ray halo extending over a few degrees was discovered by MILAGRO, HAWC and later by H.E.S.S., and by Fermi-LAT in the GeV band. Similar emission has been detected for other middle-aged pulsars in their late evolution stages, and is most plausibly explained by inverse Compton scattering of CMB and interstellar photons by relativistic electrons and positrons. These observations pose a number of theoretical challenges. Tackling these questions requires constraining the ambient magnetic field properties, which can be achieved through X-ray observations. If gamma-ray halos originate from a distribution of highly energetic electrons, synchrotron losses in the ambient magnetic fields of the same particles are expected to produce a diffuse X-ray emission with a similar spatial extension. I will present the most comprehensive multi-wavelength study of the Geminga pulsar halo to date. In addition to gamma rays, we use archival X-ray data from XMM-Newton and NuSTAR, covering a broad bandwidth (0.5-79 keV) and large field of view (~4 degrees) for the first time. We find no significant emission and set robust constraints on the X-ray halo flux. These are translated to stringent constraints on the ambient magnetic field strength and the diffusion coefficient by using a physical model considering particle injection, diffusion and cooling over the pulsar's lifetime, which is tuned by fitting multi-wavelength data. Our novel methodology for modelling and searching for synchrotron X-ray halos can be applied to other pulsar halo candidates.

Fe McBride (Bowdoin College); Nur Schettino, Owen Chase

Ghost-particle stories: Which AGN types are haunting us with neutrinos?

Neutrino (Ghost particles) have been detected by IceCube for more than a decade, which contains a flux of astrophysical neutrinos. Due to the reconstruction of particle trajectories, the angular uncertainties of neutrino events remain large. Another complication is the production of neutrinos in the atmosphere which do not necessarily point back to their source of origin. Jetted active galaxies and TDEs have been shown to be promising counterparts for some neutrino events including IceCube-35 (and PKS 1424-418), IceCube-170922A (and TXS 0506+056), the TDE AT2019dsg. A surprising result has been the increasing evidence for a connection between radio-quiet AGN and neutrino events, including IceCube-190331A (and 2MASS J22292559-20184) and NGC 1068. Here, I present new results from IceCube-200615A and IceCube220303A, both neutrino events which are consistent with radio-quiet AGN.

Courtney McIntosh (NASA GSFC / Howard University); Myles Pope, Jordan Eagle, Joel Coley

Analysis of Fermi-LAT Observations of the High-Mass Gamma-Ray Binary 1FGL J1018.6-5856

We report on Fermi observations of the High-mass Gamma-ray binary 1FGL J1018.6-5856. Using more than 15 years of Fermi-LAT data, a 15% increase from the most recent study, we find that the spectral shape of its emission hardens before and during the pre-periastron spike, which implies more efficient particle acceleration. Moving forward, we will cross-correlate the Fermi-LAT aperture photometry light curve to the radio (ATCA) and X-ray (NuSTAR and XMM-Newton) light curves in order to study the emission characteristics of the binary in greater detail.

Kaya Mori (Columbia University); Luca Abu-El Haj, Jooyun Woo, Shuhan Zhang

Applying a Genus topological test to Fermi-LAT data: searching for GeV counterparts of Galactic PeVatrons and pulsar halos

We apply a genus topological test to Fermi-LAT imaging data as an efficient method to search for extended GeV sources. The genus test, a powerful tool for identifying topological patterns in 2D images, has been previously used to search for non-Gaussian features in CMB data and to characterize the emission patterns of molecular clouds and supernova remnants. The genus test can serve as a search engine for detecting clumpy features in Fermi-LAT data and guide more rigorous but time-consuming statistical methods. In addition to validating the method through simulations, our study focuses on finding GeV counterparts of Galactic PeVatron candidates and TeV pulsar halos recently detected by LHAASO and HAWC. We also plan to conduct a genus topological survey of the Galactic Plane, which may uncover GeV clumps linked to pulsars and molecular clouds. We present our simulation results and preliminary analysis using Fermi-LAT data.

Kenichi Nishikawa (Alabama A&M University); K Hirotani (IAA), A. Meli (NCAT), C. Köhn (DTU), I. Dutan (ISS), Y. Mizuno (Shanghai Jiao Tong Univ.), N. MacDonald (MPIfR), O. Kobzar (Crarow Univ. of Technology), J. Gomez (IAA-CSIC), R. Krasnoplsky (IAA), H. Shang (IAA)

3D PIC simulations for relativistic jets with a toroidal magnetic field and results from 2D GRPIC simulations

of astrophysical plasma jets, i.e., how a toroidal magnetic field affects the evolution of pair and electron-ion jets associated to the acceleration of particles. We show that Weibel, mushroom, and kinetic Kelvin-Helmholtz instabilities excited at the linear stage, generate a quasi-steady electric field component along the jet, which accelerates and decelerates electrons. We find that the two different jet compositions (pair and electron-ion) generate different instability modes respectively and observe significant differences in the structure of the strong electromagnetic fields that are driven by the kinetic instabilities in particular with the pair jet. Moreover, the magnetic field in the non-linear stage generated by different instabilities is dissipated and reorganized into new topologies. A 3D magnetic field topology depiction indicates possible reconnection sites in the non-linear stage where the particles are significantly accelerated by the dissipation of the magnetic field associated to a possible reconnection manifestation.

Bindu Rani (NASA Goddard); Jeff Hodgson

Characterizing the variability of Fermi Blazars

Fermi observations have transformed AGN temporal studies at GeV energies, revealing novel phenomena. Power Spectrum Density (PSD) analysis, offering frequency-domain representation, provides unique insights into variability timescales and robustly characterizes underlying variability. Therefore, PSD has been extensively utilized in X-ray astronomy. Now, with 15 years of Fermi data accessible, it's the right moment to employ this technique in gamma-ray astronomy. In this talk, we will present the outcomes of our investigation into 3C 279 and other bright Fermi blazars.

Paul Ray (Naval Research Laboratory); LAT Collaboration

The Third Fermi Large Area Telescope Catalog of Gamma-Ray Pulsars

We present the Third Fermi LAT Catalog of Gamma-Ray Pulsars, a compilation of 294 pulsars found in GeV data from the Fermi LAT. We will summarize the primary results from the catalog and provide some updates on progress since the catalog was frozen.

Soebur Razzaque (University of Johannesburg and the George Washington University); Hassan Abdalla, Markus Böttcher, Justin Finke and Alberto Dominguez

Effects of cosmic voids on the propagation of VHE gamma rays and the spectra of GRB 221009A

The recent detection of gamma-ray burst GRB 221009A has attracted attention due to its record brightness and first-ever detection of > 10 TeV gamma rays from a GRB. Despite being the second-nearest GRB ever detected, at a redshift of z=0.151, the distance is large enough for severe attenuation of gamma-ray flux at these energies due to gamma-gamma pair production with the extragalactic background light (EBL). We have investigated whether the presence of cosmic voids along the line of sight can significantly impact the detectability of VHE gamma rays from distant sources. We find that the gamma-gamma opacity for VHE gamma rays can be reduced by approximately 10% and up to 30% at around 13 TeV, the highest-energy photon detected from GRB 221009A, for intervening cosmic voids along the line-of-sight with a combined radius of 110 Mpc, typically found from voids catalogs, and 250 Mpc, respectively. This reduction is substantially higher for TeV photons compared to GeV photons, attributable to the broader target photon spectrum that TeV photons interact with. This finding implies that VHE photons are more susceptible to variations in the EBL spectrum, especially in regions dominated by cosmic voids. Our study sheds light on the detection of > 10 TeV photons from GRB 221009A in particular, and on the detection of extragalactic VHE sources in general.

Alba Rico (Clemson University); Alberto Dominguez, Pablo Penil, Marco Ajello, Sara Buson, Sagar Adikhari, Maryam Movahedifar

Singular Spectrum Analysis of Fermi-LAT Blazar Light Curves: A Systematic Search for Periodicity

Blazars exhibit variable emissions across all electromagnetic spectrum, observable on various time scales. In some sources repeated variations have been observed, such as those discovered in PG 1553+113 on the scale of years. QPOs can be produced by several astrophysical mechanisms, with the possibility of binary supermassive black holes, being a compelling one, potentially revealing an interesting connection between galaxy mergers and jet physics. In this talk, we will present a novel methodology, applied for the first time in this field, using the singular spectrum analysis (SSA) on a large sample of Fermi Large Area Telescope blazars. The SSA effectively isolates periodicity by powerfully characterizing trends and underlying noise processes. This analysis identifies 46 blazar candidates for QPOs, including 24 new candidates not previously reported. This constitutes the largest sample of blazar candidates for QPOs to date, surpassing previous studies by a substantial margin.

Mallory Roberts (Eureka Scientific); Deven Bhakta, Scott Ransom

A GBT S Band Search of Low Latitude Fermi Sources for Highly Scattered Pulsars

We searched 20 unidentified Fermi sources at low Galactic latitudes for highly scattered young pulsars in the Northern sky. We performed ~2 hour observations of each source with the Green Bank Telescope using the S band receiver, choosing sources whose positional uncertainties in 4FGL-DR3 are well covered by the GBT Beam at 2GHz. Systematic uncertainties due to source confusion have previously prevented efficient searches at these higher frequencies. We discovered no new young pulsars in this survey. MSP searches are ongoing. We discuss the implications of this survey for the Galactic population of gamma-ray sources.

Mallory Roberts (Eureka Scientific); Scott Ransom, Maura Mclaughlin

Hard X-Ray Observations of the Redback PSR J1628-3205

We report on NuStar and Chandra X-Ray observations of the PSR J1628-3205 redback system. The hard X-ray emission is similar to other redbacks in having orbital modulation centered around pulsar inferior conjunction, a spectrum that is too hard for standard Fermi shock acceleration suggesting acceleration through magnetic reconnection with some softening further away from inferior conjunction. Looking at the 8 redbacks which have been observed by NuStar, there is an indication of softer spectra the more distant the shock is in terms of pulsar light cylinder radii. This may be due to partial dissipation of the magnetically dominated striped wind before the shock, leading to a mix of reconnection and Fermi shock acceleration. Under this hypothesis, we use the X-ray data to make empirically derived estimates of the dissipation radius for magnetically striped pulsar winds.

Josh Rolfe (Michigan Technological University); Jordan Forman, Janeth Valverde, Cornelia Arcaro, Nazma Islam, Aryeh Brill, Tiffany Lewis, David Kalajdziovski, Rita Sambruna, Jeremy Perkins, Deirdre Horan, Elisa Prandini, Eric Perlman.; The Fermi-LAT, MAGIC and VERITAS Collaborations.

Dramatic changes in the multiwavelength emission of TeV BL Lac 1ES 1215+303

Blazar jet collimation and particle acceleration depend on energy transport through the magnetic field. Consistent, long term blazar modeling is necessary to characterize the steady emission and variability signatures. We coordinated a 15-year, multiwavelength monitoring campaign for the BL Lac object 1ES 1215+303 between MAGIC, VERITAS, Fermi-LAT and Swift. We use autoregressive inverse gamma Brill 2022 variability model, Bayesian methods and spectral analysis methods to examine the time series and spectral data. We will present the latest findings from this study, which demonstrate a dramatic decrease in the broadband emission of the source after years of continuous brightening, and estimations on the timescale of magnetic flux accumulation.

Lorenzo Scotton (The University of Alabama in Huntsville); Frédéric Piron, Frédéric Daigne

Analysis of the prompt emission of Fermi Gamma-Ray Bursts with the Internal Shock Synchrotron Model (ISSM)

The origin of the GRB prompt emission is a matter of open debate. The observed prompt spectra typically present a non-thermal component in the keV-MeV range, and possibly an additional thermal component at low energies. GRB spectra are usually reconstructed using phenomenological models, which adapt to the data with one or more components, such as the Band and the black-body functions. This work is rather based on numerical simulations of GRB prompt emission in internal shocks above the photosphere. The predicted spectra are well reproduced in the keV-MeV range by the four-parameter spectral function 'Internal Shock Synchrotron Model (ISSM)' proposed by Yassine et al. 2020. We perform time-integrated and time-resolved spectral analyses of the 460 most fluent bursts detected by Fermi-GBM in 10 years of observations. The ISSM spectral function significantly improves the goodness of the fits compared to the Band function, especially in time-integrated analysis. We compare the resulting distribution of the photon index below the peak energy with theoretical expectations in the internal shock scenario. Finally, using ISSM instead of Band to describe the non-thermal component of GRBs 100724B, 120323A and 131014A systematically reduces the significance of an additional thermal component, confirming that in this scenario the relativistic jet is initially magnetically dominated.

Ruo-Yu Shang (Barnard College); Jordan Eagle, Sara Coutiño De León, Sajan Kumar; VERITAS, Fermi-LAT, HAWC

A multiwavelength investigation of MGRO J1908+06, with Fermi-LAT, VERITAS, and HAWC

Multiwavelength synergy among Fermi-LAT, VERITAS/HESS/MAGIC, and HAWC/LHAASO across the wide range of gamma-ray energies is a crucial mean to investigate the acceleration mechanism and the particle transport within Galactic accelerators, such as pulsar wind nebulae, supernova remnants, star clusters, and TeV halos. Many of these sources have been detected with emission of photon energies > 100 TeV and display large angular extents. The large angular extents allow detailed morphological study of particle acceleration and transport but also impose analysis challenges. While Fermi-LAT, HAWC, and LHAASO can study these sources thanks to their wide fields of view, Imaging Atmospheric Cherenkov Telescopes (IACTs) are pointed instruments with a limited field of view, which poses challenges for analyzing extended sources. We present a novel background method for IACT data that enables model-independent studies of source morphology when the source extent exceeds the IACT field of view and its application to a bright leptonic PeVatron, MGRO J1908+06 with an angular diameter of ~3.6 degrees at TeV energies. Using data from Fermi-LAT, VERITAS, and HAWC, we estimate the age and magnetic field of the PWN, essential parameters for understanding particle transport in the vicinity. The implications of how particles diffuse into the interstellar medium are discussed, providing valuable insights into the origin of Galactic cosmic rays at PeV energies.

Samuel Shilling (Lancaster University/CRESST II); S. R. Oates

An intrinsic luminosity-decay correlation in GRB radio light curves

With the regular detection of Gamma-ray Burst (GRB) afterglows with Swift over the last 20 years, the number of afterglows has become sufficient to perform statistical analyses across multiple frequencies. Studies of GRB afterglows have discovered a correlation between the luminosity measured in the early stages of the afterglow and the average rate of decay past this time. This correlation has been observed at optical/UV, X-ray and GeV frequencies. These findings suggest that more luminous afterglows decay faster than less luminous ones. In our poster, we will present a correlation in the radio afterglows of GRBs between the intrinsic luminosity measured at 10 days and the average rate of decay measured from 10 days onwards, complementary to that found in previous studies. In our poster, we discuss the radio sample, compare the correlation with that discovered at other frequencies, and investigate the possible causes of the correlation.

Chris Shrader (NASA GSFC & CUA); Bindu Rani

Exploring the Disk-Jet Connection in Gamma-Ray Blazars

We explore the disk-jet relationship in a sample of gamma-ray selected Blazar AGN. Our focus is on FSRQs for which the big blue bump and broad-line region (BLR) are well contrasted with the underlying synchrotron continuum.
Gamma-ray emission is believed to emanate from Compton scattering of lower energy photons from either the accretion disk and its associated BLR and/or torus (EC) or from the synchrotron radiation field itself (SSC).
The EC scenario is likely predominant in the most luminous objects. As proxies for the disk power, we use BLR measurements - Balmer lines or rest-frame UV lines depending on source redshift - as well as blue-bump continuum fluxes.
We present the results of cross-correlation analyses between these accretion-disk emission proxies and the gamma-ray (>100-MeV) light curves obtained by the Fermi Gamma-Ray Space Telescope, typically with ~weekly sampling.

Daniel Shy (U.S. Naval Research Laboratory)

Development of the cadmium zinc TElluride Radiation Imager (TERI)

The cadmium zinc TElluride Radiation Imager, or TERI, is an instrument to space qualify large-volume 4x4x1.5 cm^3 pixelated CdZnTe (CZT) detector technology. The CZT's anode is composed of a 22x22 array of pixels while the cathode is planar. TERI will contain four of those crystals with each pixel having an energy range of 40 keV up to 3 MeV with a resolution of 1.3% full-width-at-half maximum at 662 keV. As the detectors are 3D position sensitive, TERI can Compton image events. TERI is fitted with a coded-aperture mask which permits imaging low energy photons in the photoelectric regime. TERI's primary mission is to space-qualify large-volume CZT and measure its degradation due to radiation damage in a space environment. Its secondary mission includes detecting and localizing astrophysical gamma-ray transients. TERI is manifested on DoD's STP-H10 mission for launch to the International Space Station in early 2025.

Arathi Suraj (Northeastern University); For the GRAMS Collaboration

An Overview of pGRAMS - The Prototype Flight for GRAMS

The Gamma-Ray and AntiMatter Survey (GRAMS) is a NASA Physics of the Cosmos mission that aims to observe MeV gamma rays to probe the so-called "MeV gap," an energy band with a dearth of astrophysical observations. GRAMS also aims to measure low-energy cosmic-ray antinuclei produced through dark matter self-annihilation pathways in an effort to achieve essentially background-free dark matter searches. With a cost-effective, large-scale liquid argon time projection chamber (LArTPC) detector, GRAMS can provide an order of magnitude improvement to MeV gamma-ray sensitivity while allowing for the exploration of new dark matter parameter spaces via antinuclei measurements. Here, our detector offers a dual function, both as a Compton camera for gamma rays and an antinucleus detector.
We are currently preparing for a prototype flight (pGRAMS) scheduled for 2025/2026, and a small-scale LArTPC has been tested in our lab. pGRAMS will be a one-day balloon flight to demonstrate our detector performance in flight. In this presentation, I will discuss the GRAMS overview, detection concepts, and the current status of the pGRAMS preparation.

Donggeun Tak (Seoul National University)

Multi-wavelength afterglow analysis of the extremely bright GRB 221009A

GRB221009A is one of the most powerful gamma-ray bursts observed in the broad energy band from radio to TeV energy bands, providing invaluable information for a comprehensive understanding of the GRB afterglow. Due to its unusual brightness, this GRB allows to perform the multi-wavelength analysis with optical, X-ray, and gamma-ray data up to about 2 days after the trigger. We focused on investigating the evolution of the afterglow emission by studying the two time intervals, 0.5-0.8 and 1.2-2.1 days after the GRB trigger time. Our broadband spectral analysis across ten energy decades provides not only constraints on the physical parameters of synchrotron emission, but also reveals a clear rise of the cooling-break energy in time. The results are completely consistent with the external forward shock model with the electron spectral index of p = 2.29 ± 0.02 in the wind-like medium. This study confirms that the sychrotron process can explain the multi-wavelength afterglow emission and its evolution.

Christo Venter (Centre for Space Research, North-West University); Andreas Kopp, Zorawar Wadiasingh, Alice Harding, Matthew Baring

Broadband emission from spider binaries

About 50 'spider' binary systems, in which a millisecond pulsar heats and ablates its low-mass companion via its intense pulsar wind, are now known. This is a substantial fraction of the pulsar population seen by the Fermi Large Area Telescope (LAT). These intriguing systems exhibit radio eclipses, optical light curves from a heated companion, as well as non-thermal X-ray and GeV light curves and spectra. We have updated our code that models spider binaries by including an updated synchrotron kernel, broken power-law injection spectrum, and more realistic intrabinary shock geometries. Our updated spectral and energy-dependent light curve outputs may aid in constraining particle energetics, wind properties, shock geometry, and system inclination of several spider binaries.

Tonia Venters (NASA Goddard Space Flight Center); AMEGO-X Science Team

Probing the Multimessenger Nature of AGNs with AMEGO-X

Active galactic nuclei (AGNs) harbor intense environments that are ripe for efficient particle acceleration; however, whether they accelerate cosmic rays and produce neutrinos remains a long-standing mystery. Recently, multimessenger observations in gamma rays and neutrinos culminated in the 2017 joint detection of a flare from blazar TXS 0506+056. Additionally, the IceCube Observatory has reported observations of neutrino excesses associated with TXS 0506+056 (occurring within a 5-month period in 2014-2015) and with the radio-quiet AGN NGC 1068. While still tentative (≲ 4 sigma), these observations have provided the strongest evidence to date of a possible connection between AGNs and neutrinos. Further confirmation of this connection will require more joint observations with gamma rays, for which the medium-energy band is the best indicator as the radiation fields required for efficient neutrino production make the source opaque to higher-energy gamma rays. The All-sky Medium Energy Gamma-ray Observatory eXplorer (AMEGO-X) is a Compton-pair telescope that will feature unprecedented sensitivity in the medium-energy gamma-ray band (100 keV — >1 GeV), excellent localization capability, and polarization sensitivity at energies between 100 keV and 1 MeV. AMEGO-X observations of AGNs will be crucial to probing their multimessenger nature.

Zorawar Wadiasingh (UMD College Park / NASA GSFC); Konstantinos Kalapotharakos, Hoa Dinh, M. G. Baring, K. Hu, G. Younes, A. K. Harding, E. Gogus, C. Kouveliotou

New Physical Models of Magnetar Burst Fireballs

Magnetars are the most magnetized neutron stars with magnetic fields of the order of ~ 10^14 - 10^16 G. Magnetar short bursts (0.01-1 s long) are high-energy transients peaking in the hard X-rays (~ 30-50 keV) originating from confined hot plasmas in magnetar magnetospheres, and Fermi-GBM has detected many thousands over its lifetime. They are of significant interest to current and future high-energy instruments (e.g. NICER, IXPE, STROBE-X, HEX-P) and wide field monitors such as COSI and AMEGO-X. Interest in the physics of magnetar short bursts has intensified recently with their association with Fast Radio Bursts. We report on new energy-dependent magnetar burst fireball models accounting for curved spacetime, radiative transport in strong magnetic fields, and photon splitting. This is accomplished via a fully general relativistic ray tracing formalism accounting for local emission anisotropy and emergent spectra at burst photospheres accommodated assuming local thermal equilibrium within flux tubes for different physical descriptions of the photon gas. Radiative transport in the burst photosphere is handled via a suite of magnetized photosphere models, accounting for the polarization state and energy of photons relative to the local cyclotron resonance. The spectro-temporal lags, and transfer functions (familiar within the black hole community) are reported in a magnetar context, as multiple gravitationally-lensed images of burst regions may result depending on location and viewing geometry. Future fitting of bursts in our models could enable geometric constraints on magnetars (e.g. viewing angle and magnetic obliquity) and their bursts, as well as perhaps constraints on magnetar masses and radii.

Haocheng Zhang (UMBC/CRESST II/NASA GSFC)

X-ray to gamma-ray polarization in inhomogeneous relativistic plasma

X-ray to gamma-ray polarization, owing to the high photon energy, can probe the extreme particle acceleration sites in many astrophysical systems, such as blazars, gamma-ray bursts, and pulsar wind nebulae. Previous works often consider a homogeneous emission region in the relativistic plasma. However, the recent IXPE observations of the blazars Mrk 421 and 501 show clear evidence that the emission region is not homogeneous. Instead, these discoveries favor an energy-stratification model, which is a kind of multi-zone inhomogeneous model. This poster will revisit the X-ray to gamma-ray polarization theories under a multi-zone physical picture. We will highlight main differences in theoretical predictions from the previous one-zone models, and show how they can affect the multi-wavelength radiation and polarization signatures.

Haocheng Zhang (UMBC/CRESST II/NASA GSFC)

Probing particle acceleration mechanisms with multi-wavelength radiation and polarization signatures

Blazars are among the most powerful extragalactic cosmic particle accelerators. The unresolved blazar zone, which locates somewhere around one parsec from the central engine, can produce highly variable multi-wavelength flares, and is generally believed to be one of the main particle acceleration sites in blazars. Shock, magnetic reconnection, and turbulence can all produce the typically observed blazar spectra and light curves, but they require drastically different physical conditions in the blazar zone. This poster presents first-principle-integrated particle-in-cell and radiation transfer simulations to study the multi-wavelength radiation and polarization signatures from magnetic reconnection and turbulence. We find that they can be clearly distinguished by analyzing the multi-wavelength flux and polarization variability. This provides us a novel way to understand the particle acceleration mechanisms and physical conditions in the blazar zone.

Haocheng Zhang (UMBC/CRESST II/NASA GSFC)

Kink-driven transient quasi-periodic oscillations in blazars

The blazar zone is a unique location in the blazar jet that dissipates a large amount of jet bulk energy to accelerate nonthermal particles and produce multi-wavelength flares. Both shock and magnetic reconnection can efficiently accelerate particles, but they require drastically different jet physical conditions. Despite decades of observational and theoretical efforts, which mechanism plays a major role and the underlying jet physics remain unclear. Kink instabilities are a type of magnetohydrodynamic plasma effect that can happen in a considerably magnetized environment. Our comprehensive numerical simulations show that kink instabilities can drive correlated transient quasi-periodic oscillations (QPOs) in both flux and polarization if they are present in the blazar zone. Fermi Light Curve Repository can be a great place to look for such QPOs. If confirmed in both flux and polarization, such transient QPOs can be strong evidence for kink instabilities in the blazar zone. In this situation, the blazar zone can be unambiguously identified as considerably magnetized, and the magnetic reconnection is the main particle acceleration mechanisms in the blazar zone.

Bei Zhou (Fermilab & KICP @ UChicago); Marc Kamionkowski, José Luis Bernal, Elena Pinetti

The unresolved isotropic gamma ray background and cosmological large scale structure

We study the cross correlation between the unresolved isotropic gamma ray background detected by Fermi-LAT and the cosmological large scale structure… These correlations inform us the origin of the isotropic gamma ray background, including both astrophysical and dark matter contributions.

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11th International Fermi Symposium - Talk Abstracts

11^th International Fermi Symposium - Talk Abstracts