The Effect of Blazar Spectral Breaks on the Blazar Contribution to the Extragalactic Gamma-ray Background

TL;DR

This study models blazar spectral contributions to the extragalactic gamma-ray background using broken power laws, fitting to Fermi data, and finds that blazars could still dominate the background even with spectral breaks.

Contribution

It introduces a spectral modeling approach for blazar contributions to the EGB considering spectral breaks and fits these models to Fermi data, exploring implications for blazar dominance.

Findings

Blazar spectral breaks can still fit the EGB data well.

A dominant blazar contribution to the EGB cannot be excluded.

Featureless EGB spectra do not rule out multiple source classes.

Abstract

The spectral shapes of the contributions of different classes of unresolved gamma-ray emitters can provide insight into their relative contributions to the extragalactic gamma-ray background (EGB) and the natures of their spectra at GeV energies. We calculate the spectral shapes of the contributions to the EGB arising from BL Lacertae type objects (BL Lacs) and flat-spectrum radio quasars (FSRQs) assuming blazar spectra can be described as broken power laws. We fit the resulting total blazar spectral shape to the Fermi Large Area Telescope measurements of the EGB, finding that the best-fit shape reproduces well the shape of the Fermi EGB for various break scenarios. We conclude that a scenario in which the contribution of blazars is dominant cannot be excluded on spectral grounds alone, even if spectral breaks are shown to be common among Fermi blazars. We also find that while the observation of a featureless (within uncertainties) power-law EGB spectrum by Fermi does not necessarily imply a single class of contributing unresolved sources with featureless individual spectra, such an observation and the collective spectra of the separate contributing populations determine the ratios of their contributions. As such, a comparison with studies including blazar gamma-ray luminosity functions could have profound implications for the blazar contribution to the EGB, blazar evolution, and blazar gamma-ray spectra and emission.