Long-term X-Ray Outburst in the TeV-detected Blazar Mrk 501 in 2021-2022: Further Clues for the Emission and Unstable Processes

This paper presents the results of a detailed X-ray timing and spectral analysis of Mrk 501, which are based mainly on the Swift data obtained during 2021 February-2022 December. The source showed strongly enhanced X-ray activity, characterized by a long-term increase in the baseline 0.3-10 keV flux level superimposed by shorter-term flares on timescales of a few weeks to about 2 months. During some time intervals, Mrk 501 was the brightest blazar in the X-ray sky and, moreover, showed an intense intraday variability, which was sometimes detected within the exposures lasting a few hundred seconds. These instances were characterized by fractional variability amplitudes of 4%-18% and were mostly observed during short-term X-ray flares. The source exhibited extreme spectral properties with dominance of the spectral curvature, frequent occurrence of hard photon indices in the 0.3-10 keV and 0.3-300 GeV bands, and the peak of a synchrotron spectral energy distribution in the hard X-ray range. These properties demonstrate the importance of relativistic magnetic reconnection, first-order Fermi mechanism within the magnetic field of different confinement efficiencies, stochastic acceleration, and hadronic processes. The X-ray and γ-ray fluxes showed a log-normal distribution, which hints at the imprint of accretion disk instabilities on the blazar jet, as well as the possible presence of hadronic cascades and random fluctuations in the particle acceleration rate. The optical-UV and γ-ray variabilities showed a weak or absent correlation with the X-ray flaring activity, which is not consistent with simple leptonic models and requires more complex particle acceleration, emission, and variability scenarios.