Kapanadze, B.B.KapanadzeVERCELLONE, STEFANOSTEFANOVERCELLONEROMANO, PatriziaPatriziaROMANOHughes, P.P.HughesAller, M.M.AllerAller, H.H.AllerKharshiladze, O.O.KharshiladzeTabagari, L.L.Tabagari2020-11-182020-11-1820180004-637Xhttp://hdl.handle.net/20.500.12386/28420We 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 <SUB>p</SUB> 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<SUP>-1</SUP>. 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.STAMPAenArticle10.3847/1538-4357/aabbac2-s2.0-85044287513000431683700002https://iopscience.iop.org/article/10.3847/1538-4357/aabbac2018ApJ...858...68KFIS/05 - ASTRONOMIA E ASTROFISICA