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

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.

5 pages, 3 figures. A&A Letters, Accepted for publication

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• Probing the Transition Between the Synchrotron and Inverse-compton Spectral Components of 1ES 1959+650
• Study of correlation between optical flux and polarization variations in BL Lac objects
• The large gamma-ray flare of the FSRQ PKS 0346-27
• H.E.S.S. detection and multi-wavelength study of the $z\sim$ 1 blazar PKS 0346$-$27
• 20 years of monitoring: PKS 2155-304 and PKS 1510-089 in the eyes of Swift and Fermi. I. The case of PKS 2155-304