Contribution to the Extragalactic Gamma-ray Background from the Cascades of Very-high Energy Gamma Rays

TL;DR

This paper models how electromagnetic cascades initiated by very-high-energy gamma rays from blazars influence the extragalactic gamma-ray background, revealing significant enhancements at high energies and differences between blazar sub-populations.

Contribution

It provides the first detailed calculation of cascade radiation effects on the collective gamma-ray intensity from blazars, considering different sub-populations and EBL models.

Findings

Cascade radiation significantly boosts high-energy blazar intensity.

BL Lacs show more enhancement than FSRQs due to harder spectra.

Results depend on blazar luminosity functions and EBL models.

Abstract

As very-high--energy photons propagate through the extragalactic background light (EBL), they interact with the soft photons and initiate electromagnetic cascades of lower energy photons and electrons. The collective intensity of a cosmological population emitting at very-high energies (VHE) will be attenuated at the highest energies through interactions with the EBL and enhanced at lower energies by the resulting cascade. We calculate the cascade radiation created by VHE photons produced by blazars and investigate the effects of cascades on the collective intensity of blazars and the resulting effects on the extragalactic gamma-ray background. We find that cascade radiation greatly enhances the collective intensity from blazars at high energies before turning over due to attenuation. The prominence of the resulting features depends on the blazar gamma-ray luminosity function, spectral index distribution, and the model of the EBL. We additionally calculate the cascade radiation from the distinct spectral sub-populations of blazars, BL Lacertae objects (BL Lacs) and flat-spectrum radio quasars (FSRQs), finding that the collective intensity of BL Lacs is considerably more enhanced by cascade radiation than that of the FSRQs due to their harder spectra. As such, studies of the blazar contribution to the EGRB by Fermi will have profound implications for the nature of the EBL, the evolution of blazars, and blazar spectra.