Modeling Gamma-Ray Attenuation in High-Redshift GeV Spectra

TL;DR

This paper models the attenuation of high-energy gamma rays from distant sources by the evolving UV background, using cosmological simulations and radiative transfer to predict gamma-ray opacity up to redshift 9.

Contribution

It introduces two models for quasar emissivity and combines them with star formation models to predict gamma-ray attenuation, linking UV background evolution with gamma-ray observations.

Findings

Gamma-ray opacity increases significantly from redshift 1 to 4.

Predicted gamma-ray attenuation can be tested with Fermi and ground-based telescopes.

Constraints on early star formation and UV sources can be derived from high-redshift gamma-ray observations.

Abstract

We present two models for the cosmological UV background light, and calculate the opacity of GeV gamma--rays out to redshift 9. The contributors to the background include 2 possible quasar emissivities, and output from star--forming galaxies as determined by recent a semi--analytic model (SAM) of structure formation. The SAM used in this work is based upon a hierarchical build-up of structure in a $\Lambda$CDM universe and is highly successful in reproducing a variety of observational parameters. Above 1 Rydberg energy, ionizing radiation is subject to reprocessing by the IGM, which we treat using our radiative transfer code, CUBA. The two models for quasar emissivity differing above z = 2.3 are chosen to match the ionization rates observed using flux decrement analysis and the higher values of the line-of-sight proximity effect. We also investigate the possibility of a flat star formation rate density at z $>5$. We conclude that observations of gamma--rays from 10 to 100 GeV by Fermi (GLAST) and the next generation of ground based experiments should confirm a strongly evolving opacity from $1<$ z $<4$. Observation of attenuation in the spectra of gamma--ray bursts at higher redshift could constrain emission of UV radiation at these early times, either from a flat or increasing star-formation density or an unobserved population of sources.