Variability pattern and the spectral evolution of blazars

The rapid, large-amplitude variability and very wide spectral energy distribution are marked features of blazars. To investigate the nature of these extreme particle accelerators, a number of multi-frequency campaigns have been conducted so far. Recent campaigns of the TeV blazars, Mrk 421 and Mrk 501, revealed a correlated flare between the keV X-ray and TeV γ-ray bands, providing valuable information for the emission mechanism of blazars. However, theoretical efforts lag behind the observations. Most of the previous works consider only the steady state emission, in spite of rapid blazars' variability. To understand the time evolution of blazar spectra quantitatively, we developed a time-dependent model based on the synchrotron self-Compton (SSC) emission in a homogeneous region, incorporating the radiative cooling and acceleration process properly. In this paper, we apply this model to the X-ray flare of PKS 2155-304 observed in 1994. We obtained the satisfactory fit for both spectra and light curves with the magnetic field B ∼ 0.1 - 0.2 G and the region size R ∼ 10^-2 pc, assuming a Doppler beaming factor of δ ∼ 20 - 30.