A Suzaku X-ray Study of the Particle Acceleration Processes in the Relativistic Jet of Blazar Mrk 421

Abstract

We report on the findings of a 364 ks observation of the BL LAC object Mrk 421 with the X-ray observatory Suzaku. The analysis in this paper uses fluxes and hardness ratios (HRs) in the broad energy range from 0.5 keV to 30 keV. During the course of the observation, the 0.5-30 keV flux decreased by a factor of ~2 and was accompanied by several large flares occurring on timescales of a few hours. We find that fitting a broken power model to spectra from isolated epochs during the observation describes the data well. Different flares exhibit different spectral and HR evolutions. The cumulative observational evidence indicates that the particle acceleration mechanism in the Mrk 421 jet produces electron energy distributions with a modest range of spectral indices and maximum energies. We argue that the short-timescale X-ray spectral variability in the flares can be attributed mostly to intrinsic changes in the acceleration process, dominating other influences such as fluctuations in the Doppler beaming factor, or radiative cooling in or outside the acceleration zone.