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The 2009 Fermi Symposium

Probing Extragalactic Media with Very High Energy Gamma Rays from Cosmologically Distant Blazars

Timothy C. Arlen


Very High Energy (VHE) gamma rays (E 100 GeV) emitted from the blazar class of AGN interact with the diffuse far-IR to UV extragalactic background light (EBL) in intergalactic space, producing an electron-positron pairs. These leptons, deflected by the intergalactic magnetic field (IGMF) in turn inverse Compton (IC) scatter CMB and EBL photons, rapidly radiating away their energy and initiating a cascade of GeV-TeV photons which can then be observed by space-based Fermi Gamma-ray Space Telescope and ground-based imaging atmospheric Cherenkov telescopes (IACTs) such as VERITAS. By modeling these intergalactic cascading processes which result in spectral, angular, and temporal changes of the intrinsic characteristics of VHE emission from a blazar it appears possible to constrain parameters of the currently poorly understood IGMF, as well as possibly provide limits on the EBL spectral energy density in the far-IR to UV region. Results of computer simulations of cascading in intergalactic space which utilizes the fully relativistic description of both pair production and IC scattering in the expanding universe will be presented.