2024ApJS..275...23K

We present the spectral and timing results obtained during the intense observations of Mrk 421 by the Swift-based ultraviolet to X-ray instruments during 2018 April-2023 December. The source showed various strengths of X-ray flaring activity, exceeding a level of 3.5 × 10^–9 erg cm^–2 s^–1 during the strongest 0.3-10 keV flares. Our study identifies a number of intraday brightness variability, including 61 instances that occurred within 1 ks 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 0.3-10 keV and 0.3-300 GeV bands, with the peak of synchrotron spectral energy distribution E_p detected at the energies beyond 29 keV 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. Our spectral results also demonstrate the importance of the first-order Fermi mechanism within the magnetic field of different confinement efficiencies, stochastic acceleration, transitions in the turbulence spectrum, and hadronic cascades. The X-ray, UV, and γ-ray fluxes showed a lognormal variability, which hints at the imprint of accretion disk instabilities on the blazar jet, as well as the possible presence of hadronic cascades. The UV and γ-ray variabilities demonstrated weak correlations with the X-ray flaring activity, which is not consistent with simple synchrotron self-Compton models and requires more complex particle acceleration and emission scenarios.