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 .