We have studied the rapid X-ray variability of three extragalactic TeV γ-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 103 to 108 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∼-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∼-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 ≥ 1017 cm distant from the base of the jet, which corresponds to ≥ 102 Schwarzschild radii for 107-10 M⊙ black hole systems.
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