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