2008A&A...488..549P

Our Galaxy hosts at its dynamical center SgrA*, the closest supermassive black hole. Surprisingly, its luminosity is several orders of magnitude lower than the Eddington luminosity. However, the recent observations of occasional rapid X-ray flares from SgrA* provide constraints on the accretion and radiation mechanisms at work close to its event horizon. Our aim is to investigate the flaring activity of SgrA* and to constrain the physical properties of the X-ray flares. In Spring 2007, we observed SgrA* with XMM-Newton with a total exposure of ∼230 ks. We have performed timing and spectral analysis of the new X-ray flares detected during this campaign. To study the range of flare spectral properties, in a consistent manner, we have also reprocessed, using the same analysis procedure and the latest calibration, archived XMM-Newton data of previously reported rapid flares. The dust scattering was taken into account during the spectral fitting. We also used Chandra archived observations of the quiescent state of SgrA* for comparison. On April 4, 2007, we observed for the first time within a time interval of roughly half a day, an enhanced incidence rate of X-ray flaring, with a bright flare followed by three flares of more moderate amplitude. The former event represents the second brightest X-ray flare from Sgr A* on record with a peak amplitude of about 100 above the quiescent luminosity. This new bright flare exhibits similar light-curve shape (nearly symmetrical), duration (∼3ks) and spectral characteristics to the very bright flare observed in October 3, 2002 by XMM-Newton. The measured spectral parameters of the new bright flare, assuming an absorbed power law model taken into account dust scattering effect, are N_H=12.3^+2.1_–1.8x10²²cm^–2 and Γ=2.3±0.3 calculated at the 90% confidence level. The spectral parameter fits of the sum of the three following moderate flares, while lower (N_H=8.8^+4.4_–3.2x10²²cm^–2 and Γ=1.7^+0.7_–0.6), are compatible within the error bars with those of the bright flares. The column density found, for a power-law model taking into account the dust scattering, during the flares is at least two times higher than the value expected from the (dust) visual extinction toward SgrA* (A_V∼25mag), i.e., 4.5x10²²cm^–2. However, our fitting of the SgrA* quiescent spectra obtained with Chandra, for a power-law model taking into account the dust scattering, shows that an excess of column density is already present during the non-flaring phase. The two brightest X-ray flares observed so far from SgrA* exhibited similar soft spectra.