X-ray hiccups from Sagittarius A* observed by XMM-Newton^*

The second brightest flare and three moderate flares caught in half a day

¹ Observatoire astronomique de Strasbourg, Université Louis-Pasteur, CNRS, INSU, 11 rue de l'Université, 67000 Strasbourg, France e-mail: porquet@astro.u-strasbg.fr
² Max-Plank-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
³ Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
⁴ CEA, IRFU, Service d'Astrophysique, 91191 Gif-sur-Yvette, France
⁵ Astroparticule et Cosmologie, 10 rue Alice Domont et Léonie Duquet, 75205 Paris Cedex 13, France
⁶ Department of Physics and Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
⁷ Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
⁸ XMM-Newton Science Operations Centre, ESA, Villafranca del Castillo, Apartado 78, 28691 Villanueva de la Canada, Spain

Received: 16 April 2008
Accepted: 16 June 2008

Abstract

Context. Our Galaxy hosts at its dynamical center Sgr A*, 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 Sgr A* provide constraints on the accretion and radiation mechanisms at work close to its event horizon.

Aims. Our aim is to investigate the flaring activity of Sgr A* and to constrain the physical properties of the X-ray flares.

Methods. In Spring 2007, we observed Sgr A* 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 Sgr A* for comparison.

Results. 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 (~3 ks) 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 =  cm^-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 =  cm^-2 and ), 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 Sgr A* (A_V ~ 25 mag), i.e., 4.5 10²² cm^-2. However, our fitting of the Sgr A* 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.

Conclusions. The two brightest X-ray flares observed so far from Sgr A* exhibited similar soft spectra.