Infrared to X-ray observations of PKS 2155-304 in a low state

TL;DR

This study analyzes infrared to X-ray observations of the blazar PKS 2155-304 during a low state, revealing shifts in emission peaks and spectral features that inform models of high-energy gamma-ray production.

Contribution

It provides new insights into the spectral behavior of PKS 2155-304 during a low state, highlighting the movement of the synchrotron peak and the spectral break characteristics.

Findings

Synchrotron peak shifted from ultraviolet to optical wavelengths.

X-ray spectrum fitted with a broken power law with a break at about 3.5 keV.

Soft X-rays linked to synchrotron emission, hard X-rays to between synchrotron and inverse-Compton humps.

Abstract

AIMS: Our goal is to understand the nature of blazars and the mechanisms for the generation of high-energy gamma-rays, through the investigation of the prototypical blazar PKS 2155-304, which shows complex behaviour. METHODS: We analyze simultaneous infrared-to-X-ray observations obtained with XMM-Newton and REM on November 7, 2006, when the source was in a low X-ray state. We perform a comparative analysis of these results with those obtained from previous observations in different brightness states. RESULTS: We found that the peak of the synchrotron emission moved from ultraviolet to optical wavelengths and the X-ray spectrum is best fit with a broken power law model with Gamma_2 ~ 2.4 harder than Gamma_1 ~ 2.6 and a break at about 3.5 keV. This suggests that the soft X-rays (E < 3.5 keV) are related to the high-energy tail of the synchrotron emission, while the hard X-rays (E > 3.5 keV) are from the energy region between the synchrotron and inverse-Compton humps. The different variability at energies below and above the break strengthens this hypothesis. Our results also stress the importance of monitoring this source at both low and high energies to better characterize its variability behaviour.