A&A 488, 549-557 (2008)
DOI: 10.1051/0004-6361:200809986

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

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

D. Porquet¹, N. Grosso¹, P. Predehl², G. Hasinger², F. Yusef-Zadeh³, B. Aschenbach², G. Trap^{4, 5}, F. Melia⁶, R. S. Warwick⁷, A. Goldwurm^{4, 5}, G. Bélanger⁸, Y. Tanaka², R. Genzel², K. Dodds-Eden², M. Sakano⁷, and P. Ferrando<sup>4, 5

1</sup>  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 =  cm^-2 and = 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 ( =  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* ( ~ 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.

Key words: Galaxy: center -- X-rays: individuals: Sgr A* -- X-rays: general -- radiation mechanisms: general

© ESO 2008

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