On the duty-cycle of γ-ray blazars

Abstract

We study several properties of blazars detected in the γ-ray energy range by comparing the Energetic Gamma Ray Experiment Telescope (EGRET) sources with a sample of radio blazars which can be considered possible γ-ray candidates. We define three classes: non-γ-ray blazars, blazars with quasi-steady γ-ray emission and γ-ray blazars with substantial activity level. We find that, on average, BL Lacertae objects show a relatively steady γ-ray emission, when detected. On the other hand, most flat-spectrum radio quasars (FSRQs) show substantial γ-ray variability. We attribute a γ-ray activity index ψ = ψ -7 x 10 -7 cm -2 s -1 to all EGRET blazars, and show that FSRQs dominate the sample with non-zero ψ in the range 0 < ψ -7 < 0.035. By combining the information of detected and candidate active galactic nuclei, we characterize the blazar activity, including the discovery of a region of consistency between the γ-ray flaring duty-cycle and the recurrence time between flares. We also find a possible relation between the activity index of FSRQs and their black hole mass. More optical and γ-ray data are crucially important to test this relation.