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 γ-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 Γ 2 ∼ 2.4 harder than Γ 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.

PKS 2005-489 at VHE: four years of monitoring with HESS and simultaneous multi-wavelength observations

“…In this data set, its presence becomes detectable because of the steepness and very low flux of the synchrotron (X-ray) spectrum. A similar behavior has recently been observed in the HBL PKS 2155-304 during a low state (Foschini et al 2008;Zhang 2008). This transition between steep synchrotron and flat inverse-Compton spectra is instead fully visible inside the observed X-ray passband for intermediate BL Lac objects (e.g.…”

Simultaneous multiwavelength observations of the second exceptionalγ-ray flare of PKS 2155–304 in July 2006

“…10 and Table 4, during all times the X-ray spectrum of PKS 2155-304 remains steep, with a convex shape and no signs of flattening at high energies (as instead found in XMM-Newton observations performed in November 2006; Foschini et al 2008;Zhang 2008). This means that the peak of the synchrotron emission has not entered the observed energy range at any time, and that there is no sign of the possible emergence of the IC component in the hard X-ray band.…”

Fermi/LAT detection of extraordinary variability in the gamma-ray emission of the blazar PKS 1510-089

“…Other hints of changes in the blazar color were reported for PKS 2155−304 (z = 0.116) in 2006 November (Foschini et al 2008, but no γ-ray data were available) and PKS 1510−089 (z = 0.36) in 2009 March (Abdo et al 2010;D'Ammando et al 2011). In the latter case, the peak of the synchrotron shifted from infrared to optical/UV frequencies and the source was detected at hundreds of GeV by Abramowksi et al (2013), although the X-ray spectrum did not change (Abdo et al 2010).…”