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We have used all 20 archival XMM-Newton observations of PKS 2155-304 withsimultaneous X-ray and UV/optical data to study its long term flux and spectralvariability. We find significant variations, in all bands, on time-scales ofyears with an rms amplitude of ~ 35-45 per cent, through the optical/UVvariations are not correlated with those in the X-ray. We constructed SEDs thatspan more than three orders of magnitude in frequency and we first fitted themwith a log-parabolic model; such models have been applied many times in thepast for this, and other, blazars. These fits were poor, so we then examinedcombined power-law and log-parabolic fits that are improvements. These modelsindicate that the optical/UV and X-ray flux variations are mainly driven bymodel normalization variations, but the X-ray band flux is also affected byspectral variations, as parametrized with the model 'curvature' parameter, b.Overall, the energy at which the emitted power is maximum correlates positivelywith the total flux. As the spectrum shifts to higher frequencies, the spectral'curvature' increase, in contrast to what is expected if a single log-parabolicmodel were an acceptable representation of the broad band SEDs. Our resultssuggest that the optical/UV and X-ray emissions in this source may arise fromdifferent lepton populations.

Spectral energy distributions of the BL Lac PKS 2155-304 from XMM-Newton