Astrophysikalisches Seminar

The TeV-detected nearby (z = 0.034) blazar Mrk 501 is prominent by the extreme X-ray and TeV-band variability on diverse timescales ranging from years down to a few minutes.  In the flaring epochs,  X-ray spectra were frequently very and extremely hard with the photon indices of 1.4--1.8, the synchrotron SED peak E_p situated beyond 2 keV and sometimes at the energies higher than 10 keV. In turn, this requires extreme electron energy distribution (EED) with the high-energy cut-off extending to Lorentz factors of 10^6. The source frequently showed a transition from log-parabolic to hard power-law EED and conversely within 1-ks time intervals, hinting at the presence of turbulence-driven magnetic reconnection in the relativistically magnetized jet area with very small spatial extents of ~10^12 cm. The log-normal variability demonstrated by the source in the 300 MeV—300 GeV band could be contributed by the hadronic cascades (along with the imprint of the disc-related instable processes and random fluctuations in the particle acceleration rate), the presence of which explains the frequent occurrence of the hard , very hard and extremely hard LAT-band spectra easier than the leptonic models: the significant fraction of the spectra were presented with the photon index Gamma<1.8 down to Gamma~1.3. The hadronic processes could also lead to the frequent absence of the correlation between LAT (or FACT) band variability with those observed in the synchrotron frequency range.