X-ray variability in BL Lac objects

Abstract

X-ray properties of BL Lac objects are classified into two categories. One is low energy peaked BL Lacs (LBLs). The X-ray spectrum of LBLs is a power law type with an index of about 1.7 and is flatter than those in optical and UV bands. X-ray flux varies in correlated way with optical flux for a major outburst. The other catgory is high-energy peaked BL Lacs (HBLs), whose X-ray spectra are either single power law type or power law type with gradual steepening toward higher energy. The spectra are steeper than those of LBLs and can be connected smoothly from optical and UV bands. X-rays from HBLs are characterized by correlated flux variation with spectral index and soft lag. The spectra becomes steeper with decreasing flux, and the peak or bottom of the light curve in the soft band is delayed from those in hard band. These properties are interpreted by a synchrotron-cooling model, and cooling time, escape time, and injection time of synchrotron emitting electrons are determine! d by fitting the light curves with the model. The flux increase and hardening of X-rays during the rise of the outburst was expressed by Fermi acceleration of low energy electrons and hard lag of the start time of the burst was strong support for the acceleration. On the other hand, flat X-ray spectrum of LBLs is attributed to inverse Compton scattering of internal and/or external seed photons.