Derivation of a Relation for the Steepening of TeV Selected Blazar Gamma-ray Spectra with Energy and Redshift

TL;DR

This paper derives a relation describing how blazar gamma-ray spectra steepen with energy and redshift, separating intrinsic source effects from intergalactic absorption, enabling potential insights into the intergalactic background light evolution.

Contribution

It introduces a new relation for spectral steepening that distinguishes intrinsic and intergalactic effects, aiding IBL evolution studies independently of stellar models.

Findings

The relation accurately models spectral steepening in observed blazars.

Application to Fermi data constrains intergalactic background light evolution.

Potential to determine IBL evolution without relying on stellar evolution models.

Abstract

We derive a relation for the steepening of blazar gamma-ray spectra between the multi-GeV Fermi energy range and the TeV energy range observed by atmospheric Cerenkov telescopes. The change in spectral index is produced by two effects: (1) an intrinsic steepening, independent of redshift, owing to the properties of emission and absorption in the source, and (2) a redshift-dependent steepening produced by intergalactic pair production interactions of blazar gamma-rays with low energy photons of the intergalactic background light (IBL). Given this relation, with good enough data on the mean gamma-ray SED of TeV selected BL Lacs, the redshift evolution of the IBL can, in principle, be determined independently of stellar evolution models. We apply our relation to the results of new Fermi observations of TeV selected blazars.