The X-ray variability of Mrk 421

The results of monitoring the Haring activity of Mrk 421 with the XMM-Newton and RXTE orbiting observatories are presented. The ASM/RXTE data show that, during most of the observational period, 2002-2006, the source was in a very active state, with a mean 1.5-12 keV flux equals similar to 17 mCrab. Analysis of daily and weekly averaged data shows strong X-ray variability of Mrk 421 on time scales from days to months. The long-term light curve demonstrates a rich variety of variability patterns and a complex hierarchical flare structure, in most cases superimposed on longer trends of rising and decreasing source activity. Quantitative estimates of the variability of Mrk 421 shows evidence of correlations between the source intensity in X-rays and the variability parameters, both for separate years and epochs and for most of the observational period. The soft and hard X-ray components (according to XMM-Newton data) are correlated with each other, and spectral changes (according to ASM/RXTE data) are characterized by increasing hardness with brightening of the object in X-rays. Moderate spectral variability at individual epochs and evidence for an energy dependence of the source variability are also observed. The 2-12 keV fluxes from Mrk 421 detected by XMM-Newton are consistent with the maximum 2-10 keV flux detected by ASCA similar to 10 years ago. An analysis of simultaneous day-time-scale variability of Mrk 421 in X-rays (ASM/RXTE) and very-high-energy gamma-rays (GT-48 Cherenkov telescope of the Crimean Astrophysical Observatory) in periods of increased activity of the source in December 2002 and April 2004 is also presented.