Abstract:
We have used all 20 archival XMM–Newton observations of PKS 2155 − 304 with simulta-
neous 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 magni-
tude 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 normaliza-
tion 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 frequen-
cies, 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.
