Absorption of High‐Energy Gamma Rays by Interactions with Extragalactic Starlight Photons at High Redshifts and the High‐Energy Gamma‐Ray Background

Abstract

In this paper, we extend previous work on the absorption of high energy γ-rays in intergalactic space by calculating the absorption of 10 to 500 GeV γ-rays at high redshifts. This calculation requires the determination of the high-redshift evolution of the intergalactic starlight photon field, including its spectral energy distribution out to frequencies beyond the Lyman limit. To estimate this evolution, we have followed a recent analysis of Fall, Charlot & Pei, which reproduces the redshift dependence of the starlight background emissivity obtained by the Canada-France redshift survey group. We also include the UV background from quasars.We give our results for the γ-ray opacity as a function of redshift out to a redshift of 3. We also give predicted γ-ray spectra for selected blazars and extend our calculations of the extragalactic γ-ray background from blazars to an energy of 500 GeV with absorption effects included. Our results indicate that the extragalactic γ-ray background spectrum from blazars should steepen significantly above 20 GeV, owing to extragalactic absorption. Future observations of a such a steepening would thus provide a test of the blazar origin hypothesis for the γ-ray background radiation. We also note that our absorption calculations can be used to place limits on the redshifts of γ-ray bursts; for example, our calculated opacities indicate that the 17 Feb. 1994 burst observed by EGRET must have originated at z ≤∼ 2. Finally, our estimates of the high-energy γ-ray background spectrum are used to determine the observability of multi-GeV γ-ray lines from the annihilation of supersymmetric dark-matter particles in the galactic halo.

Luminous and high-frequency peaked blazars: the origin of the γ-ray emission from PKS 1424+240

“…This absorption effect increases with the distance and with the energy of the VHE photon: the result is a softening of blazar spectra for increasing redshifts, and, finally, a limit on the detection of distant blazars above 100 GeV (Salamon & Stecker 1998). On the other hand, this absorption can be used to put constraints on the EBL itself and indeed, the VHE detection of the first blazars at redshifts z ≥ 0.15 imposed strong constraints on early EBL models (Aharonian et al 2006).…”

Constraining extragalactic background light from TeV blazars

“…The method of galaxy counts is used to estimate the EBL, but it only provides a lower limit owing to the unknown of unresolved sources. Various models for the EBL have been published: Salamon & Stecker (1998), Malkan & Stecker (1998, , Kneiske et al (2002Kneiske et al ( , 2004, Primack et al (2005Primack et al ( , 2008, Gilmore et al (2008Gilmore et al ( , 2009, Franceschini et al (2008), Razzaque et al (2009), Finke et al (2010). These models include different degrees of complexity, observational constraints, and data inputs.…”

Implications of cosmological gamma-ray absorption

“…Method (iii) computes the MRF directly from the global SFR inferred from tracers of cosmic chemical evolution, such as the various Lyman α systems (Salamon & Stecker 1998;Pei et al 1998), or from deep galaxy surveys (e.g., Madau et al 1998;Rowan-Robinson 2001;Franceschini 2000). The spectral energy distribution (SED) for the globally averaged stellar population residing in galaxies can be estimated with population synthesis models (e.g.…”