BL Lacertae contribution to the extragalactic gamma-ray background

Free access article

Issue		A&A Volume 479, Number 1, February III 2008


Page(s)		41 - 47
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20065605

A&A 479, 41-47 (2008)
DOI: 10.1051/0004-6361:20065605

BL Lacertae contribution to the extragalactic gamma-ray background

T. M. Kneiske^{1, 2} and K. Mannheim¹

¹ Universität Würzburg, Am Hubland, 97057 Würzburg, Germany
² Department of Physics and Mathematical Physics, University of Adelaide, SA 5005, Australia
e-mail: tkneiske@physics.adelaide.edu.au

(Received 15 May 2006 / Accepted 28 September 2007)

Abstract
Very high energy gamma-rays from blazars traversing cosmological distances through the metagalactic radiation field can convert into electron-positron pairs in photon-photon collisions. The converted gamma-rays initiate electromagnetic cascades driven by inverse-Compton scattering off the microwave background photons. Using a model for the time-dependent metagalactic radiation field consistent with all currently available far-infrared-to-optical data, we calculate the cascade contribution from faint, unresolved high- and low-peaked blazars to the extragalactic gamma-ray background as measured by EGRET. For low-peaked blazars, we adopt a spectral index consistent with the mean spectral index of EGRET detected blazars, and the EGRET luminosity function. For high-peaked blazars, we adopt template spectra matching prototype sources observed with air-Cherenkov telescopes up to 30 TeV, and a luminosity function based on X-ray measurements. The low number of $\sim$ 20 for nearby high-peaked blazars with a flux exceeding $10^{-11}~\rm cm^{-2}\,s^{-1}$ above 300 GeV inferred from the luminosity function is consistent with the results from air-Cherenkov telescope observations. Including the cascade emission from higher redshifts, the total high-peaked blazar contribution to the observed gamma-ray background at GeV energies can account for ~30%.

Key words: galaxies: BL Lacertae objects: general -- cosmology: diffuse radiation -- gamma rays: observations

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