Variability Pattern and the Spectral Evolution of the BL Lacertae Object PKS 2155−304

The TeV blazar PKS 2155-304 was monitored with the X-ray satellite ASCA in1994 May, as part of a multiwavelength campaign from the radio to X-ray bands.At the beginning of the two-day continuous observation, we detected a largeflare, where the 2-10 keV flux changed by a factor of 2 on a time scale of 3 x10^4 sec. During the flare, the increase in the hard X-ray flux clearlypreceded that observed in the soft X-rays, the spectral evolution tracking a`clockwise loop' in the flux versus photon index plane. Ascribing theenergy-dependent variability to differential synchrotron cooling ofrelativistic electrons, we estimate the magnetic field B in the emissionregion. We tested two different methods of comparing the time series in variousX-ray bands: (i) fitting the light curves to a Gaussian and searching for thetime shift of the peak of the flare, and (ii) calculating the discretecorrelation function. Both methods yielded a consistent solution of B ~ 0.1 G.We also found that the flare amplitude becomes larger as the photon energyincreases, while the duration of the flare stays roughly constant through theASCA energy band (0.7-7.5 keV). In the framework of the time dependentsynchrotron self-Compton model in a homogeneous region, we consider a flarewhere the maximum Lorentz factor (Gmax) of the injected electrons increasesuniformly throughout the emission volume. The temporal evolution of spectra aswell as the light curves were reproduced with the physical parametersself-consistently determined from seven observables. We obtain B ~ 0.1-0.2 Gand a region size R ~ 10^-2 pc, for relativistic beaming with a Doppler factorof D ~ 20-30. We discuss the significance of light travel time effects.Comment: 43 pages, 8 figures, accepted for publication in the Astrophysical Journa