Evidence for a characteristic time-scale in the X-ray light curves of TeV blazars

Abstract

We have studied the rapid X-ray variability of four TeV blazars, Mrk 421, Mrk 501, PKS 2155−304 and 1ES 2344+514. Analyzing the X-ray light curves obtained from ASCA and/or RXTE observations, we have investigated the variability in the time domain from 103 to 107 sec. The structure functions show a roll-over around 1 day, which can be interpreted as the typical time-scale of day-by-day flare events. On time-scales shorter than 1 day, variability is significantly suppressed, indicating a steep power spectrum density (PSD) of f−2∼−3. This is very different from other types of mass-accreting black-hole systems for which the short timescale variability is well characterized by a fractal, flickering noise (f−1∼−2). Importantly, the steep PSD index and the characteristic time-scale of day-by-day flares imply that the X-ray emitting site in the jet is of limited spatial extent; D≃1017–18 cm distant from the base of the jet, which corresponds to ≃102–4Rg for 107–9M⊙ black-hole systems.