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 targ et of two intense multiwavelength campaigns: in 1991 November and in 1994 M ay. Although the spectral energy distributions at both epochs were quite si milar, the source exhibited two very distinct variability patterns that can not 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-rays and UV of similar to 3 hr, while during the second epoc h the variability amplitude increased as a function of wavelength, with the EUV flare peaking similar to 1 day after the X-ray flare. We model the sou rce using a time-dependent inhomogeneous accelerating jet model. We reprodu ce the general characteristics of the different variability signatures by a ssuming that plasma disturbances with different physical properties propaga te downstream in an underlying jet characterized by the same set of physica l parameters at both epochs. A time delay of similar to 1 day between the h ardening of the UV spectral index and the UV flux, present at both epochs, is modeled with stochastic fluctuations in the particle acceleration manife sted through small variations of the maximum energy of the injected electro ns. We predict that similar time delays will be present in future observati ons, even in the absence of strong variability events. We stress the import ance of observations at neighboring frequencies as a diagnostic tool for th e structure of the quiescent jet in blazars, especially in the seemingly du ll case in which strong variability is absent.