On the duty-cycle of γ-ray blazars

We study several properties of blazars detected in the gamma-ray energy range by comparing the Energetic Gamma Ray Experiment Telescope (EGRET) sources with a sample of radio blazars which can be considered possible gamma-ray candidates. We define three classes: non-gamma-ray blazars, blazars with quasi-steady gamma-ray emission and gamma-ray blazars with substantial activity level. We find that, on average, BL Lacertae objects show a relatively steady gamma-ray emission, when detected. On the other hand, most flat-spectrum radio quasars (FSRQs) show substantial gamma-ray variability. We attribute a gamma-ray activity index psi = psi(-7) x 10(-7) cm(-2) s(-1) to all EGRET blazars, and show that FSRQs dominate the sample with non-zero psi in the range 0 < psi(-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 gamma-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 gamma-ray data are crucially important to test this relation.