BL Lacertae Objects and the Extragalactic Gamma-Ray Background

TL;DR

This study finds a strong correlation between gamma-ray and radio emissions in BL Lac objects, estimates their gamma-ray duty cycle, and concludes they contribute about 45-77% to the extragalactic gamma-ray background.

Contribution

It provides the first gamma-ray luminosity function of BL Lac objects derived from radio data and gamma-ray duty cycle estimates, linking them to the extragalactic gamma-ray background.

Findings

BL Lac objects contribute approximately 45% to the EGRB.

Estimated gamma-ray duty cycle for BL Lac objects is about 0.11.

BL Lac objects and radio quasars together account for about 77% of the EGRB.

Abstract

A tight correlation between gamma-ray and radio emission is found for a sample of BL Lacertae (BL Lac) objects detected by Fermi Gamma-ray Space Telescope (Fermi) and the Energetic Gamma-Ray Experiment Telescope (EGRET). The gamma-ray emission of BL Lac objects exhibits strong variability, and the detection rate of gamma-ray BL Lac objects is low, which may be related to the gamma-ray duty cycle of BL Lac objects. We estimate the gamma-ray duty cycle ~ 0.11, for BL Lac objects detected by EGRET and Fermi. Using the empirical relation of gamma-ray emission with radio emission and the estimated gamma-ray duty cycle, we derive the gamma-ray luminosity function (LF) of BL Lac objects from their radio LF. Our derived gamma-ray LF of BL Lac objects can almost reproduce that calculated with the recently released Fermi bright active galactic nuclei (AGN) sample. We find that about 45% of the extragalactic diffuse gamma-ray background (EGRB) is contributed by BL Lac objects. Combining the estimate of the quasar contribution to the EGRB in the previous work, we find that about 77% of the EGRB is contributed by BL Lac objects and radio quasars.