Study of the X-ray activity of Sgr A* during the 2011 XMM-Newton campaign

International audienceSgr A* is the closest supermassive black hole (˜ 4 × 10^6 M_⊙) located at the dynamical center of our galaxy. It has a very low bolometric luminosity (˜ 9.4 × 10^{-9} times the Eddington luminosity) and, consequently, a very low mass accretion rate (˜ 10^{-6} M_⊙/yr) but flaring activity can be observed in near-infrared, X-ray, sub-millimeter and radio. To constrain the origin of such events, it is important to investigate the timing and spectral properties of these flares, especially in X-rays. During the 2011 XMM-Newton campaign (whose total exposure was ˜226 ks) in coordination with the 1.3 mm Very-Long-Baseline Interferometry array, two X-ray flares have been observed in the 2-10 keV energy band. To perform the timing analysis of the light curves, we apply the Bayesian-blocks method to the XMM-Newton event lists, using a two-step algorithm to correct for any detector flaring background. Furthermore, we compute X-ray smoothed light curves in order to have better accuracy on the substructures and the amplitude of the flares. The first X-ray flare was observed on March 30, 2011 with a peak amplitude of about 3 times the non-flaring level. It is characterized by two sub-flares: the first one is very short (˜458 s) with an unabsorbed peak-luminosity of ˜ 9.4 × 10^{34} erg s^{-1}, whereas the second one is longer (˜1542 s) with a lower unabsorbed peak- luminosity (˜ 6.8 × 10^{34} erg s^{-1}). The waiting time between the two sub-flares (˜1000 s) is one of the smallest ever observed. If we compare this value with those observed during the 2012 Chandra XVP campaign, we can favor the hypothesis that this event is a single flare rather than two distinct sub-flares. We developed a hotspot model to explain the double-peaks shape of the light curve of this flare with the gravitational lensing and Doppler boosting. However, the decrease of the flux back to the quiescent level between the two substructures cannot be satisfactorily reproduced with this simple model. This observation allows us to reject this flaring model even when it is made slightly more complex than a simple hotspot. The very rapid flux variation during the first sub-flare allow us to constrain the distance and the size of the flaring source. Since the proper time around a supermassive black hole is always longer than the observed duration due to the time dilatation in strong gravity field and assuming that the rise and decay phases are due to magnetic energy heating and synchrotron cooling of infrared photons, respectively, we derive a range to the radial distance of 4-100(+19,-29) rg with rg=0.04 AU. The corresponding source radii at this distance are 1.8-2.87±0.01 rg