Bulletin of the American Physical Society
APS April Meeting 2014
Volume 59, Number 5
Saturday–Tuesday, April 5–8, 2014; Savannah, Georgia
Session S4: Invited Session: Cosmological Constraints from Gamma-Ray Data |
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Sponsoring Units: DAP Chair: Julie McEnery, NASA Room: Chatham Ballroom C |
Monday, April 7, 2014 1:30PM - 2:06PM |
S4.00001: DAP Young Star: Intensity and Origin of the Extragalactic Gamma-ray Background Invited Speaker: Keith Bechtol The gamma-ray sky can be decomposed into individually detected sources, diffuse emission attributed to the interactions of cosmic rays with gas and radiation fields in our Galaxy, and a residual all-sky emission component commonly called the isotropic diffuse gamma-ray background (IGRB). The IGRB comprises all extragalactic emissions too faint or too diffuse to be resolved in a given survey, as well as any residual Galactic foregrounds that are indistinguishable from isotropic. The sum of the IGRB and individually resolved extragalactic sources represents our best estimate of the total extragalactic gamma-ray background. The first IGRB measurement with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope used 10 months of sky-survey data and considered an energy range between 200 MeV and 100 GeV. Improvements in event selection and characterization of particle backgrounds, better understanding of the diffuse Galactic emission, updated emission models for the Earth atmosphere, Sun, and Moon, as well as a longer data accumulation of 50 months, allow for a refinement and extension of the IGRB measurement with the LAT, now covering the energy range from 100 MeV to 820 GeV. We discuss the possible presence of a high-energy cutoff ($>$100 GeV) in the IGRB, as well as systematic uncertainties that impact the shape and normalization of the measured spectrum. Finally, we review the current census of extragalactic source populations and truly diffuse processes contributing to the extragalactic gamma-ray background. [Preview Abstract] |
Monday, April 7, 2014 2:06PM - 2:42PM |
S4.00002: Using Very High Energy Photons from Blazars for Cosmological Insight Invited Speaker: Amy Furniss Gamma-ray blazars are among the most extreme astrophysical sources, harboring energetic phenomena far beyond that attainable by terrestrial accelerators. These galaxies are understood to be active galactic nuclei that are powered by accretion onto supermassive black holes and have relativistic jets pointed along the Earth line of sight. The very high energy photons emitted by these extragalactic sources are detectable with ground based imaging atmospheric Cherenkov telescopes such as VERITAS, MAGIC and HESS. As these photons propagate extragalactic distances, the interaction with the diffuse starlight that pervades the entire Universe results in a distance and energy dependent gamma-ray opacity, offering a unique method for probing photon densities on cosmological scales. These galaxies have also been postulated to be potential sources of ultra-high-energy cosmic rays, a theory which can be examined through the deep gamma-ray observations of sources which probe moderate gamma-ray opacities. If confirmed as cosmic ray progenitors, these galaxies would provide an opportunity to probe the intergalactic magnetic field, as the charged particles would be deflected from the line of sight in a field-dependent manner. [Preview Abstract] |
Monday, April 7, 2014 2:42PM - 3:18PM |
S4.00003: Cascade gamma rays as a probe of the high-energy universe: general constraints, hints, and implications Invited Speaker: Kohta Murase Very-high-energy (VHE) and ultra-high-energy (UHE) gamma rays from extragalactic sources experience electromagnetic cascades during their propagation in intergalactic space. Recent observations by Fermi and Cherenkov telescopes allow us to get more insight into VHE and UHE gamma-ray emitters. The latest Fermi data on the diffuse gamma-ray background have provided us with powerful constraints on potential cosmic-ray sources and annihilating/decaying dark matter, as well as a possible hint of the cascades. The relevance of the cascades is also motivated by some extreme blazars seen in the VHE range, and understanding such VHE gamma-ray sources is relevant to constrain intergalactic magnetic fields in voids and identify extragalactic cosmic-ray sources. [Preview Abstract] |
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