The Large Flares of Mrk 421 in 2006: signature of electron acceleration and energetic budget of the jet

摘要

We present the results of a deep spectral analysis of all Swift observationsof Mrk 421 between April 2006 and July 2006, when it reached its highest X-rayflux recorded until the end of 2006. We completed this data set with otherhistorical X-ray observations. We used the full data set to investigate thecorrelation between the spectral parameters. We found a signature of stochasticacceleration in the anticorrelation between the peak energy (Ep) of thespectral energy distribution (SED) and the spectral curvature parameter (b). Wefound signature of energetic budget of the jet in the correlation between thepeak flux of the SED (S p) and Ep. Moreover, using simultaneous SwiftUVOT/XRT/BAT data, we demonstrated, that during the strongest flares, theUV-to-X-ray emission from Mrk 421 requires that the curved electrondistribution develops a low energy power-law tail. The observed spectralcurvature and its anticorrelation with Ep is consistent with both stochasticacceleration or energy-dependent acceleration probability mechanisms, whereasthe power-law slope of XRT-UVOT data is close to that inferred from the GRBsX-ray afterglow and in agreement with the universal first-order relativisticshock acceleration models. This scenario implies that magnetic turbulence mayplay a twofold role: spatial diffusion relevant to the first order process andmomentum diffusion relevant to the second order process.