Swift obs. of Mrk 421 in selected epochs. II.

Best fits of the 0.3-10keV spectra were mostly obtained using the log-parabolic model showing the relatively low spectral curvature that is expected in the case of efficient stochastic acceleration of particles. The position of the synchrotron spectral energy density peak E_p_ of 173 spectra is found at energies higher than 2keV.

The photon index at 1 keV exhibited a very broad range of values a=1.51-3.02, and very hard spectra with a<1.7 were observed during the strong X-ray flares, hinting at a possible hadronic jet component. The spectral parameters varied on diverse timescales and showed a correlation in some periods, which is expected in the case of first- and second-order Fermi acceleration. The 0.3-10keV flux showed strong X-ray flaring activity by a factor of 3-17 on timescales of a few days-weeks between the lowest historical state and that corresponding to a rate higher than 100ct/s.