Spectral Evolution of PKS 2155-304 observed with BeppoSAX during an Active $\gamma$-ray Phase

We present the results of BeppoSAX observations of PKS 2155-304 during an intense gamma-ray flare. The source was in a high X-ray state. A temporal analysis of the data reveals a tendency of the amplitude of variations to increase with energy, and the presence of a soft lag with a timescale of the order 10^3 s. A curved continuum spectrum, with no evidence of spectral features, extends up to ~50 keV, while there is indication of a flatter component emerging at higher energies, consistent with the interpretation of the broad band spectral energy distribution (SED) as due to synchrotron self-Compton (SSC) emission from a single region. Notably, the fitting of the SED with such a model is consistent with an interpretation of the detected soft lag as due to radiative cooling, supporting the idea that radiation losses play an important role in variability. The observed shifts of the SED peaks between the lowest and highest flux levels can be accounted for by an increase of the break energy in the relativistic particle spectrum. The model predicts emission at TeV energies in good agreement with the recently reported detection