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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21 February 2005
HIGH ENERGY GAMMA-RAY ASTRONOMY: 2nd International Symposium on High Energy Gamma-Ray Astronomy
26-30 July 2004
Heidelberg (Germany)
Research Article|
February 21 2005
BL Lac Contribution to the Extragalactic Gamma‐Ray Background
Tanja M. Kneiske;
Tanja M. Kneiske
*Universitaet Wuerzburg, Am Hubland, 97074 Wuerzburg, Germany
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Karl Mannheim
Karl Mannheim
*Universitaet Wuerzburg, Am Hubland, 97074 Wuerzburg, Germany
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AIP Conf. Proc. 745, 578–584 (2005)
Citation
Tanja M. Kneiske, Karl Mannheim; BL Lac Contribution to the Extragalactic Gamma‐Ray Background. AIP Conf. Proc. 21 February 2005; 745 (1): 578–584. https://doi.org/10.1063/1.1878466
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