BL Lac Contribution to the Extragalactic Gamma-Ray Background

Details BL Lac Contribution to the Extragalactic Gamma-Ray Background BL Lac Contribution to the Extragalactic Gamma-Ray Background

Feb 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005aipc..745..578k&link_type=abstract

HIGH ENERGY GAMMA-RAY ASTRONOMY: 2nd International Symposium on High Energy Gamma-Ray Astronomy. AIP Conference Proceedings, Vo

Astronomy and Astrophysics

Astronomy

3

Active And Peculiar Galaxies And Related Systems, Gamma-Ray Sources, Gamma-Ray Bursts, Background Radiations

Scientific paper

Very high energy gamma-rays (Eγ > 20 GeV) 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 to faint gamma-ray halos. Using a model for the time-dependent metagalactic radiation field consistent with all currently available far-infared-to-optical data, we compute (i) the expected gamma-ray attenuation in blazar spectra, and (ii) the cascade contribution from faint, unresolved blazar 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.

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