Modeling the Variability of the BL Lacertae Object PKS 2155−304

The bright X-ray selected BL Lacertae object PKS 2155-304 has been the targetof two intense multiwavelength campaigns, in November 1991 and in May 1994.Although the spectral energy distributions at both epochs were quite similar,the source exhibited two very distinct variability patterns that cannot beeasily reconciled with homogeneous, one-zone jet models. During the first epochthe variability was almost achromatic in amplitude, with a time lag betweenX-rays and UV of $\approx 3$ h, while during the second epoch the variabilityamplitude increased as a function of wavelength, with the EUV flare peaking$\approx 1$ day after the X-ray flare. We model the source using atime-dependent inhomogeneous accelerating jet model. e reproduce the generalcharacteristics of the different variability signatures by assuming that plasmadisturbances with different physical properties propagate downstream in anunderlying jet characterized by the same set of physical parameters at bothepochs. A time delay of $\approx$ 1 day between the hardening of the UVspectral index and the UV flux, present at both epochs, is modeled withstochastic fluctuations in the particle acceleration manifested through smallvariations of the maximum energy of the injected electrons. We predict thatsimilar time delays will be present in future observations, even in the absenceof strong variability events. We stress the importance of observations atneighboring frequencies as a diagnostic tool for the structure of the quiescentjet in blazars, especially in the seemingly dull case when strong variabilityis absent.Comment: 8 pages, 5 figures. Accepted in ApJ Letter