Characteristic X-ray variability of TeV blazars: Probing the link between the jet and the central engine

We have studied the rapid X-ray variability of three extragalactic TeV gamma -ray sources : Mrk 421, Mrk 501, and PKS 2155-304. Analyzing the X-ray light curves obtained from ASCA and/or Rossi X-Ray Timing Explorer observations between 1993 and 1998, we have investigated the variability in the time domain from 10(3) to 10(8) s. For all three sources, both the power spectrum density (PSD) and the structure function (SF) show a rollover with a timescale of the order of 1 day or longer, which may be interpreted as the typical timescale of successive flare events. Although the exact shape of turnover is not well constrained and the low-frequency (long timescale) behavior is still unclear, the high-frequency (short timescale) behavior is clearly resolved. We found that, on timescales shorter than 1 day, there is only small power in the variability, as indicated by a steep power spectrum density of f(-2 similar to -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 PSD (f(-1 similar to -2)). The steep PSD index and the characteristic timescale of flares imply that the X-ray-emitting site in the jet is of limited spatial extent : D greater than or equal to 10(17) cm distant from the base of the jet, which corresponds to greater than or equal to 10(2) Schwarzschild radii for 10(7-10) M-circle dot black hole systems.