Letter to the Editor

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

¹ INAF/IASF-Bologna, via Gobetti 101, 40129 Bologna, Italy e-mail: foschini@iasfbo.inaf.it
² Dipartimento di Scienze, Università degli Studi dell'Insubria, via Valleggio 11, 22100 Como, Italy
³ INAF, Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate, Italy
⁴ Osservatorio Astronomico, Università di Perugia, via B. Bonfigli, 06126 Perugia, Italy
⁵ George Mason University, 4400 University Drive, Fairfax, VA, 22030, USA
⁶ INAF, Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
⁷ INAF, Osservatorio Astronomico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
⁸ NASA Goddard Space Flight Center, Code 661, Greenbelt, MD 20771, USA

Received: 15 April 2008
Accepted: 29 April 2008

Abstract

Aims. Our goal is to understand the nature of blazars and the mechanisms for the generation of high-energy γ-rays, through the investigation of the prototypical blazar PKS , 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 harder than and a break at about 3.5 keV. This suggests that the soft X-rays ( keV) are related to the high-energy tail of the synchrotron emission, while the hard X-rays ( 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.