BL Lac contribution to the Extragalactic Gamma-Ray Background
Abstract
Very high energy gamma-rays(E>20GeV) from blazars traversing cosmological distances through the metagalactic radiation field can convert to electron-positron pairs in photon-photon collisions. The converted gamma-rays initiate electromagnetic cascades driven by inverse-Compton scattering off the microwave background photons. The cascades shift the injected gamma ray spectrum to MeV-GeV energies. Randomly oriented magnetic fields rapidly isotropize the secondary electron-positron beams resulting from the beamed blazar gamma ray emission, leading o faint gamma-ray halo. Using a model for the time-dependent metagalactic radiation field consistent with all currently available far-infrared-to-optical data, we compute (i) the expected gamma-ray attenuation in blazar spectra, and (ii) the cascade contribution from faint, unresolved blazars to the extragalactic gamma-ray background as measured by EGRET, assuming a generic emitted spectrum extending to an energy of 10 TeV. The latter cascade contribution to the EGRET background is fed by the assumed >20 GeV emission from the hitherto undiscovered sources, and we estimate their dN-dz distribution taking into account that the nearby (z<0.2) fraction of these sources must be consistent with the known (low) numbers of sources above 300 GeV.
Cite
@article{arxiv.astro-ph/0411146,
title = {BL Lac contribution to the Extragalactic Gamma-Ray Background},
author = {Tanja Kneiske and Karl Mannheim},
journal= {arXiv preprint arXiv:astro-ph/0411146},
year = {2007}
}
Comments
6 pages, 5 figures to appear in the proceedings of the Gamma 2004 Symposium on High Energy Gamma Ray Astronomy, Heidelberg, July 2004 (AIP Proceedings Series)