BL Lacertae objects and the extragalactic γ-ray background

Abstract

A tight correlation between γ-ray and radio emission is found for a sample of BL Lacertae (BL Lac) objects detected by the Fermi Gamma-ray Space Telescope (Fermi) and the Energetic Gamma-Ray Experiment Telescope (EGRET). The γ-ray emission of BL Lac objects exhibits strong variability, and the detection rate of γ-ray BL Lac objects is low, which may be related to the γ-ray duty cycle of BL Lac objects. We estimate the γ-ray duty cycle, δγ ≃ 0.11, for BL Lac objects detected by EGRET and Fermi. Using the empirical relation of γ-ray emission with radio emission and the estimated γ-ray duty cycle δγ, we derive the γ-ray luminosity function (LF) of BL Lac objects from their radio LF. Our derived γ-ray LF of BL Lac objects can almost reproduce that calculated with the recently released Fermi bright active galactic nuclei (AGN) sample. Comparison of the derived LF of the γ-ray BL Lac objects in this work with that derived by Abdo et al. (2009a) requires the γ-ray duty cycle of BL Lac objects to be almost luminosity-independent. We find that ∼ 45% of the extragalactic diffuse γ-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 ∼ 77% of the EGRB is contributed by BL Lac objects and radio quasars.