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

TL;DR

This study analyzes long-term multi-band observations of the blazar PKS 2155-304, revealing significant flux variability, spectral complexity, and potential different emission regions for optical/UV and X-ray bands, challenging simple spectral models.

Contribution

It provides a comprehensive analysis of multi-epoch XMM-Newton data, demonstrating the limitations of log-parabolic models and proposing combined spectral fits to better understand blazar emission mechanisms.

Findings

Significant flux variability with 35-45% rms amplitude.

Poor fits with simple log-parabolic models, improved by combined models.

Optical/UV and X-ray emissions may originate from different lepton populations.

Abstract

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.