Astronomy & Astrophysics

Issue		A&A Volume 455, Number 3, September I 2006
Page(s)		871 - 877
Section		Extragalactic astronomy
DOI		10.1051/0004-6361:20064959

A&A 455, 871-877 (2006)
DOI: 10.1051/0004-6361:20064959

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

L. Foschini¹, G. Tagliaferri², E. Pian³, G. Ghisellini², A. Treves⁴, L. Maraschi², F. Tavecchio², G. Di Cocco¹ and S. R. Rosen⁵

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 $\alpha \simeq 1.8$ ( $f_\nu \propto \nu^{-\alpha}$ ), is probably due to synchrotron emission, while the harder part of the spectrum, which has $\alpha \simeq 1$ , 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.

Key words: galaxies: BL Lacertae objects: general -- galaxies: BL Lacertae objects: individual: S5 0716+714 -- X-rays: galaxies

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