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

TL;DR

This study analyzes Swift observations of Mrk 421 in 2006, revealing signatures of electron acceleration, the energetic jet budget, and the development of a low-energy electron tail during strong flares, advancing understanding of blazar jet physics.

Contribution

It provides the first detailed spectral analysis linking stochastic acceleration signatures and jet energetics in Mrk 421 using multi-instrument data.

Findings

Anticorrelation between peak energy and spectral curvature indicates stochastic acceleration.

Correlation between peak flux and peak energy reveals jet energetic budget.

During strong flares, a low-energy power-law tail develops in the electron distribution.

Abstract

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