Waiting times between gamma-ray flares of flat spectrum radio quasars, and constraints on emission processes

Abstract

Context.The physical scenario responsible for gamma-ray flaring activity and its location for flat spectrum radio quasars is still debated.Aims.The study of the statistical distribution of waiting times between flares, defined as the time intervals between consecutive activity peaks, can give information on the distribution of flaring times and constrain the physical mechanism responsible for gamma-ray emission.Methods.We adopt here a scan statistic-driven clustering method (iSRS) to recognize flaring states within theFermi-LAT archival data, and identify the time of activity peaks.Results.We obtained that waiting times between flares can be described with a Poissonian process, consisting of a set of overlapping bursts of flares, with an average burst duration of ∼0.6 year and average rate of ∼1.3 y−1. For short waiting times (below 1 d host-frame) we found a statistically relevant second population, the fast component, consisting of a few tens of cases, most of them revealed for CTA 102. Interestingly, the period of conspicuous detection of the fast component of waiting times for CTA 102 coincides with the reported crossing time of the superluminal K1 feature with the C1 stationary feature in radio.Conclusions.To reconcile the recollimation shock scenario with the bursting activity, we have to assume that plasma streams with a typical length of ∼2 pc (in the stream reference frame) reach the recollimation shock. Otherwise, the distribution of waiting times can be interpreted as originating from relativistic plasma moving along the jet for a deprojected length of ∼30−50 pc (assuming a bulk Γ = 10) that sporadically produces gamma-ray flares. In the magnetic reconnection scenario, reconnection events or plasma injection to the reconnection sites should be intermittent. Individual plasmoids can be resolved in a few favourable cases only, and could be responsible for the fast component.