An excess of cosmic positrons above 10 GeV with respect to the spallation reaction of cosmic rays (CRs) with the interstellar medium has been measured by Pamela, Fermi-LAT and with unprecedented precision by the AMS-02 experiment. Various interpretations have been invoked to interpret this excess, such as production from supernova remnants, pulsar wind nebulae (PWNe) and dark matter. A dominant contribution from dark matter is ruled out by the bounds found in gamma rays and other indirect searches. Models where supernova remnants produce secondary CRs struggle to explain the observed CR fluxes by AMS-02. Finally, severe constraints for a significant PWN contribution comes from the detection of very high-energy emission from Monogem and Geminga PWNe by Milagro and HAWC experiment. In this talk we will present a detailed study of the GeV gamma-ray halo around Geminga and Monogem, and show the constraints found for the contribution of these PWNe to the positron excess combining Milagro and HAWC data with measurements from the Fermi-LAT for the first time. We will report the detection of a significant emission from Geminga PWN derived by including the proper motion of the its pulsar. Then we will demonstrate that using gamma-ray data from the LAT is of central importance to provide a precise estimate for a PWN contribution to the cosmic positron flux. We will also present the most promising energies to detect a signal of synchrotron emission for positrons emitted by PWNe and will discuss the prospects of detecting this signature with current or future experiments.
I am currently a Researcher at the Istituto Nazionale di Fisica Nucleare in Torino (Italy). I graduated with a joint PhD program between the University’ di Torino in Italy and Univerite de Grenoble Alpes. Then I moved to USA where I have been a Postdoctoral fellow at the Stanford University for three years and a Researcher at the Catholic University of America and the NASA’s Goddard Space Flight Center. My main scientific interests are in the field of Astroparticle Physics and span from the indirect search of dark matter to the modeling of cosmic-ray acceleration from Galactic sources and propagation in the Galaxy.
Classical novae are thermonuclear explosions that occur on the surfaces of white dwarf stars in interacting binary systems. It has long been thought that the luminosity of classical novae is powered by continued nuclear burning on the surface of the white dwarf after the initial runaway. However, recent observations of GeV gamma-rays from classical novae, detected by Fermi-LAT, have hinted that shocks internal to the nova ejecta may dominate the nova emission. Shocks have also been suggested to power the luminosity of events as diverse as stellar mergers, supernovae, and tidal disruption events, but observational confirmation has been lacking. In this talk I present simultaneous space-based optical and Fermi gamma-ray observations of the 2018 nova V906 Carinae (ASASSN-18fv), revealing a remarkable series of distinct correlated flares in both bands. The optical and gamma-ray flares occur simultaneously, implying a common origin in shocks. During the flares, the nova luminosity doubles, implying that the bulk of the luminosity is shock-powered. Our data provide direct observational evidence that shocks can power substantial luminosity in classical novae and other optical transients.
Elias Aydi is a postdoctoral research associate at Michigan State University. He graduated with a PhD in Astronomy from the University of Cape Town in 2018. Elias's main research interests involve studying novae and time domain astronomy. Elias uses space-based facilities, like Fermi, and ground-based facilities to collect data across the electromagnetic spectrum for a better understanding of nova eruptions.
The active galactic nucleus TXS 0128+554 is a rare example of a nearby (z = .0365) non-blazar galaxy that has been detected in gamma-rays by the Fermi LAT instrument. We carried out multi-epoch radio imaging with the VLBA, and X-ray imaging with Chandra. The AGN is unresolved in a 19.3 ks Chandra exposure, and has a compact radio morphology (only 16 pc across) consisting of a bright core and two cocoon-like mini-lobes. The measured lobe advance speed of 0.32 ± 0.07 c indicates a kinematic age of only 82 ± 17 yr. The lobe's properties indicate that the jets have undergone episodic activity, and were relaunched a decade ago. A comparison with other Fermi-detected and non-Fermi-detected young AGN jets indicates that the gamma-ray emission likely originates in the inner jet/core region, and not the cocoon. This suggests that other nearby, recently launched AGN jets may be good candidates for detection by the Fermi-LAT instrument.
Dr. Matthew Lister is a professor at Purdue University whose research
interests include high-luminosity active galactic nuclei,
extragalactic jets, radio surveys, and Very Long Baseline
Interferometry. He currently heads a large observational program
() that uses the Very Long Baseline
Array and other telescopes to study the structures and evolutionary
changes in several hundred of the most powerful known jets in the
universe. These jets display noticeable relativistic effects,
including light aberration, time delays, and apparent superluminal
Prior to his appointment at Purdue's Department of Physics and
Astronomy, Matt served as a Karl Jansky Fellow at the National Radio
Astronomy Observatory, Charlottesville and a faculty lecturer at the
University of Virginia. He also served as a Caltech Postdoctoral
Fellow at NASA's Jet Propulsion Laboratory, as part of the U.S. Space
VLBI project. He earned a Ph.D. in Astronomy in 1999 from Boston
University, a M.Sc. degree from the University of Victoria, and a
B.Sc. degree from the University of Toronto.
Magnetars are slowly-rotating neutron stars with extremely strong magnetic fields 1013-15 G, episodically emitting 100 ms long X-ray bursts with energies of 1040-41 erg. Rarely, they produce extremely bright, energetic giant flares that begin with a short (0.2 s), intense flash, followed by fainter, longer lasting emission modulated by the magnetar spin period (typically 2-12 s), thus confirming their origin. Over the last 40 years, only three such flares have been observed in our local group, which all suffered from instrumental saturation due to their extreme intensity. It has been proposed that extra-galactic giant flares likely constitute a subset of short gamma-ray bursts, as the sensitivity of current instrumentation prevents us from detecting the pulsating tail, while the initial bright flash is readily observable out to distances of 10-20 Mpc. Here, we report X- and gamma-ray observations by the Fermi Gamma-ray Burst Monitor and Swift Burst Alert Telescope of GRB 200415A, which exhibits a rapid onset, very fast time variability, flat spectra and significant sub-millisecond spectral evolution. These attributes match well with those expected for a giant flare from an extra-galactic magnetar, noting that GRB 200415A is directionally associated with the galaxy NGC 253 (3.5 Mpc away). The detection of 3 MeV photons provides definitive evidence for relativistic motion of the emitting plasma. The observed rapid spectral evolution can naturally be generated by radiation emanating from such rapidly-moving gas in a rotating magnetar.
Originally from Wales, Dr. Oliver Roberts received his BSc in Physics and Astronomy at the University of Liverpool and Liverpool John Moores University, England in 2007. He completed his PhD in Physics at the University of York, England in 2011, before working in the field of Nuclear Physics at the University of Brighton, England until 2014. He was a post-doctoral researcher at University College Dublin, Ireland in 2014, where he was introduced to the Fermi Gamma-ray Burst Monitor (GBM), publishing papers primarily on Terrestrial Gamma-ray Flashes (TGFs) and highly magnetized stars, or Magnetars. He was awarded a NASA Post-doctoral program award to work on pulsars in Huntsville, AL in 2017, but transitioned quickly to a more permanent role as an Associate Scientist at the Universities Space Research Association (USRA) during the same year. In Huntsville, Dr. Roberts continues to study transients, as well as develop X-ray imaging optics and gamma-ray instruments for future missions. He’s received several awards as part of the GBM team, which includes the Bruno Rossi Prize in High-Energy Astrophysics in 2018, the NASA Space Flight Awareness Team Award in 2017 and the NASA Group Achievement Award in 2016. He has received several grants, and published extensively in the fields of TGF, Magnetar and GRB science. In his spare time, he helps run the Astronomy on Tap branch in Huntsville and is an avid astrophotographer and hiker.