Speaker
Description
Markarian 421 is one the most extreme blazars characterized by complex, unpredictable timing/spectral variability, exclusively strong X-ray flares in some epochs, very broad, nonthermal spectral energy distribution (SED) extending over 19 orders of the frequency and showing a typical two-"hump" structure. The lower-energy component, ranging from the radio to X-rays, is widely accepted to be a synchrotron radiation emitted by ultra-relativistic electrons/positrons/protons, while the origin of higher-energy emission (the MeV-TeV range) still remains controversial (synchrotron self-Compton, external Compton, hadronic etc.). All these mechanisms need the presence of highly-relativistic particles in the jet, to be initially accelerated via the Blandford-Znajek mechanism and magneto-hydrodynamic processes in the vicinity of the central super-massive black hole. However, particles loose the energy, sufficient for emitting the KeV photons, very quickly and the source can maintain its flaring state on the daily-weekly timescales if some additional acceleration mechanisms are continuously at work. According to different studies and simulations, the particles can gain a tremendous energy due to the propagation of relativistic shocks through the jet: by means of first-order Fermi mechanism at the shock front, or they undergo an efficient stochastic (second-order Fermi) acceleration close to the shock, in the turbulent jet medium. Our intense X-ray spectral study of Mrk 421 has revealed the dominance of these processes in different epochs: while the spectral curvature and photon index show a positive cross-correlation during some flares (expected in the framework of energy-dependent acceleration probability scenario: a particular case of first-order Fermi mechanism), other epochs clearly demonstrate the observables of the stochastic (second-order Fermi) acceleration (low spectral curvature; anti-correlation between the spectral curvature and the position of the synchrotron SED peak). During several X-ray flares, the source also showed hard power-law spectra, expected in the case of relativistic magnetic reconnection.
