Degeneracy of the SED Model of the blazar Mrk 421

Yamada, Y.,  Uemura, M.,  Itoh, R.,  Fukazawa, Y.,  Ohno, M., and Imazato, F., “Variations of the physical parameters of the blazar Mrk 421 based on analysis of the spectral energy distributions”, PASJ, 72, 42, 2020

The MCMC code for the SSC model optimization to the SED data is available at the project page of yysscfit

We report on the variations of the physical parameters of the jet observed in the blazar Mrk 421, and discuss the origin of X-ray flares in the jet, based on analysis of several spectral energy distributions (SEDs). The SEDs are modeled using the one-zone synchrotron self-Compton model, its parameters determined using a Markov chain Monte Carlo method. The lack of data at TeV energies means many of the parameters cannot be uniquely determined and are correlated. These are studied in detail. We find that the optimal solution can be uniquely determined only when we apply a constraint to one of four parameters: the magnetic field, the Doppler factor, the size of the emitting region, and the normalization factor of the electron energy distribution. The result of our SED analysis suggests that, in the X-ray faint state, the emission occurs in a relatively small area with a relatively strong magnetic field. The X-ray bright state shows a tendency opposite to that of the faint state. The presence of two kinds of emitting areas implies that the one-zone model is unsuitable for reproducing at least part of the observed SEDs.