SIMBAD references

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, though the optical/UV variations are not correlated with those in the X-ray. We constructed spectral energy distributions (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' increases, 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.