Modeling the variability of the BL Lacertae object PKS 2155-304

摘要

The bright X-ray-selected BL Lacertae object PKS 2155-304 has been the target of two intense multi- wavelength campaigns: in 1991 November and in 1994 May. Although the spectral energy distributions at both epochs were quite similar, the source exhibited two very distinct variability patterns that cannot be easily reconciled with homogeneous, one-zone jet models. During the first epoch the variability was almost achromatic in amplitude, with a time lag between X-ray and UV of ～3 hr, while during the second epoch the variability amplitude increased as a function of wavelength, with the EUV flare peaking ～1 day after the X-ray flare.