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We investigate the electron energy distributions (EEDs) and the accelerationprocesses in the jet of Mrk 421 through fitting the spectral energydistributions (SEDs) in different active states in the frame of a one-zonesynchrotron self-Compton (SSC) model. After assuming two possible EEDs formedin different acceleration models: the shock accelerated power-law withexponential cut-off (PLC) EED and the stochastic turbulence acceleratedlog-parabolic (LP) EED, we fit the observed SEDs of Mrk 421 in both low andgiant flare states by using the Markov Chain Monte Carlo (MCMC) method whichconstrains the model parameters in a more efficient way. Our calculatingresults indicate that (1) the PLC and LP models give comparably good fits forthe SED in low state, but the variations of model parameters from low state toflaring can be reasonably explained only in the case of the PLC in low state;and (2) the LP model gives better fits compared to the PLC model for the SED inflare state, and the intra-day/night variability observed at GeV-TeV bands canbe accommodated only in the LP model. The giant flare may be attributed to thestochastic turbulence re-acceleration of the shock accelerated electrons in lowstate. Therefore, we may conclude that shock acceleration is dominant in lowstate, while stochastic turbulence acceleration is dominant in flare state.Moreover, our result shows that the extrapolated TeV spectra from the best-fitSEDs from optical through GeV with the two EEDs are different. It should beconsidered in caution when such extrapolated TeV spectra are used to constrainextragalactic background light (EBL) models.

Emitting electrons spectra and acceleration processes in the jet of Mrk 421: from low state to giant flare state