Simultaneous X-ray and optical observations of S5 0716+714 after the outburst of March 2004

¹ INAF/IASF – Bologna, via Gobetti 101, 40129 Bologna, Italy e-mail: foschini@iasfbo.inaf.it
² INAF – Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
³ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy
⁴ Dipartimento di Scienze, Università degli Studi dell'Insubria, via Valleggio 11, 22100 Como, Italy
⁵ Mullard Space Science Laboratory, University College of London Holmbury St Mary, Dorking, Surrey, RH5 6NT, UK

Received: 3 February 2006
Accepted: 26 April 2006

Abstract

At the end of March 2004, the blazar S5 0716+714 underwent an optical outburst that prompted for quasi-simultaneous target-of-opportunity observations with the INTEGRAL and XMM-Newton satellites. In this paper, we report the results of the XMM-Newton and INTEGRAL OMC data analysis. The X-ray spectrum is well-represented by a concave broken power-law model, with the break at about 2 keV. In the framework of the synchrotron self-Compton model, the softer part of the spectrum, which is described by a power law of index (), is probably due to synchrotron emission, while the harder part of the spectrum, which has , is due to inverse Compton emission. The blazar shows the long and short-term variability typical of low-frequency peaked BL Lac (LBL): the former is manifested by a gradual decrease in the optical flux from the peak as observed by ground telescopes at the end of March 2004, while the latter is characterized by soft X-ray and optical flares on time scales from a few thousand seconds to few hours. We can follow spectral variations on sub-hour time scales and study their correlation with the flux variability. We find evidence that the peak energy of the time-resolved spectra is increasing with flux. The modeling of the spectral energy distribution compared with archival observations suggests that the long-term variability (from outburst to quiescence or viceversa) could be due to a change in the injected power, while the short-term variability (flares) could be explained with changes in the slope of the distribution of the electrons.