Swift Observations of Mrk 421 in Selected Epochs. II. An Extreme Spectral Flux Variability in 2009–2012

We present the results from a detailed spectral and timing study of Mrk 421 based on the rich archival Swift data obtained during 2009-2012. Best fits of the 0.3-10 keV 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 2 keV. 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 0.3-10 keV 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 100 cts s-1. Moreover, 113 instances of intraday variability were revealed, exhibiting shortest flux-doubling/halving times of about 1.2 hr, as well as brightenings by 7%-24% in 180-720 s and declines by 68%-22% in 180-900 s. The X-ray and very high-energy fluxes generally showed a correlated variability, although one incidence of a more complicated variability was also detected, indicating that the multifrequency emission of Mrk 421 could not be generated in a single zone

Swift Observations of Mrk 421 in Selected Epochs. I. The Spectral and Flux Variability in 2005–2008

We present detailed results of Swift observations of the nearby TeV-detected blazar Mrk 421, based on the rich archival data obtained during 2005 March-2008 June. The best fits of the 0.3-10 keV spectra were mainly obtained using the log-parabolic model, yielding low spectral curvatures expected in the case of the efficient stochastic acceleration of particles. During strong X-ray flares, the position of the synchrotron spectral energy distribution peak {E}{{p}} was beyond 8 keV for 41 spectra, while it sometimes was situated at the UV frequencies in quiescent states. The photon index at 1 keV exhibited a broad range, and the values a< 1.70 were observed during the strong flares, hinting at the possible presence of a jet hadronic component. The spectral parameters were correlated in some periods, expected in the framework of the first- and second-order Fermi accelerations of X-ray emitting particles, as well as in the case of turbulence spectrum. The 0.3-10 keV flux and spectral parameters sometimes showed very fast variability down to the fluctuations by 6-20% in 180-960 s, possibly related to the small-scale turbulent areas containing strongest magnetic fields. X-ray and very high-energy fluxes often showed correlated variability, although several occurrences of more complicated variability patterns are also revealed, indicating that the multifrequency emission of Mrk 421 could not be generated in a single zone