Energy dependent X-ray variability of the TEV blazars PKS 2155-304 and MKN 421.

We present the X-ray variability properties of the TeV blazars PKS 2155-304 and MKN421 as observed with BeppoSAX and ASCA. The minimum timescales of ∼ 1000s are suggested. For PKS 2155-304 we found that the soft X-ray lags (relative to higher energies) are inversely correlated with the source intensity, i.e. the lag is longer when the source is fainter. In April 1998 a flare of MKN 421 was detected, during which higher energy X-rays lagged lower energy ones by ∼ 2500s. The origin of these lags is briefly discussed. [ABSTRACT FROM AUTHOR]