Swift obs. of Mrk 421. IV. 5.5yrs obs.

The source showed various strengths of X-ray flaring activity, exceeding a level of 3.5x10^-9^erg/cm^2^/s during the strongest 0.3-10keV flares. Our study identifies a number of intraday brightness variability, including 61 instances that occurred within 1ks exposures that are consistent with the shock-in-jet scenario and accompanied by significant, fast spectral changes.

The source exhibited extreme spectral properties with dominance of the log-parabolic distributions of photons with energy and the frequent occurrence of hard photon indices in the 0.3-10keV and 0.3-300GeV bands, with the peak of synchrotron spectral energy distribution Ep detected at the energies beyond 29keV for the first time. The source showed very fast transitions of log-parabolic-to-power-law spectra, most plausibly caused by turbulence-driven relativistic magnetic reconnection.