Contribution from unresolved discrete sources to the Extragalactic Gamma-Ray Background (EGRB)

TL;DR

This paper investigates the contributions of unresolved discrete sources such as blazars, AGNs, and galaxies to the extragalactic gamma-ray background, highlighting the potential significance of non-blazar sources and the role of observational limitations.

Contribution

It provides an analysis of how various unresolved sources, including non-blazar AGNs and star-forming galaxies, contribute to the EGRB, expanding beyond blazar-focused studies.

Findings

Unresolved blazars are major contributors to the EGRB.

Non-blazar AGNs and starburst galaxies may significantly contribute.

Detection capabilities influence the observed source population.

Abstract

The origin of the extragalactic gamma-ray background (EGRB) is still an open question, even after nearly forty years of its discovery. The emission could originate from either truly diffuse processes or from unresolved point sources. Although the majority of the 271 point sources detected by EGRET (Energetic Gamma Ray Experiment Telescope) are unidentified, of the identified sources, blazars are the dominant candidates. Therefore, unresolved blazars may be considered the main contributor to the EGRB, and many studies have been carried out to understand their distribution, evolution and contribution to the EGRB. Considering that gamma-ray emission comes mostly from jets of blazars and that the jet emission decreases rapidly with increasing jet to line-of-sight angle, it is not surprising that EGRET was not able to detect many large inclination angle active galactic nuclei (AGNs). Though Fermi could only detect a few large inclination angle AGNs in the first three months' survey, it is expected to detect many such sources in the near future. Since non-blazar AGNs are expected to have higher density as compared to blazars, these could also contribute significantly to the EGRB. In this paper we discuss contributions from unresolved discrete sources including normal galaxies, starburst galaxies, blazars and off-axis AGNs to the EGRB.