X‐Ray Spectral Variability of TeV Blazars during Rapid Flares

The spectral energy distribution (SED) of TeV blazars peaks both at keV andTeV energies. The X-ray emission is generally believed to originate in thesynchrotron emission from relativistic electrons (and positrons) in the jet ofthese sources, while the origin of the gamma-ray emission is still beingdebated. We report results from a systematic study of X-ray spectralvariability of Mrk 421 and Mrk 501 during individual flares that last forseveral days, making use of some of the high-quality data that have recentlybecome available. The X-ray spectra of the two sources fall on the oppositesides of the synchrotron peak of their respective SEDs, so they together mayoffer additional insights into the physical origin of X-ray variability. Wemodeled each of the time-resolved X-ray spectra over a {\em single} flare byadopting a homogeneous spatial distribution and an instantaneous power-lawspectral distribution for the emitting particles. We focused on the variationof four key parameters: particle spectral index, maximum Lorentz factor, energydensity, and magnetic field. While there is considerable degeneracy in thefits, we show that, in order to account for the X-ray spectral variabilityobserved in Mrk 421, at least three of the parameters are required to vary inmost cases, with the spectral index being one of them. The observations of Mrk501 support the conclusion, although the quality of the data is not as good. Wediscuss the implications of the results.Comment: 17 pages, 4 figures, 4 tables, accepted by ApJ; one typo (unit of E_tot) correcte