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Title:
X-ray hiccups from Sagittarius A* observed by XMM-Newton. The second brightest flare and three moderate flares caught in half a day
Authors:
Porquet, D.; Grosso, N.; Predehl, P.; Hasinger, G.; Yusef-Zadeh, F.; Aschenbach, B.; Trap, G.; Melia, F.; Warwick, R. S.; Goldwurm, A.; Bélanger, G.; Tanaka, Y.; Genzel, R.; Dodds-Eden, K.; Sakano, M.; Ferrando, P.
Affiliation:
AA(Observatoire astronomique de Strasbourg, Université Louis-Pasteur, CNRS, INSU, 11 rue de l'Université, 67000 Strasbourg, France ), AB(Observatoire astronomique de Strasbourg, Université Louis-Pasteur, CNRS, INSU, 11 rue de l'Université, 67000 Strasbourg, France), AC(Max-Plank-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany), AD(Max-Plank-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany), AE(Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA), AF(Max-Plank-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany), AG(CEA, IRFU, Service d'Astrophysique, 91191 Gif-sur-Yvette, France ; Astroparticule et Cosmologie, 10 rue Alice Domont et Léonie Duquet, 75205 Paris Cedex 13, France), AH(Department of Physics and Steward Observatory, University of Arizona, Tucson, AZ 85721, USA), AI(Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK), AJ(CEA, IRFU, Service d'Astrophysique, 91191 Gif-sur-Yvette, France ; Astroparticule et Cosmologie, 10 rue Alice Domont et Léonie Duquet, 75205 Paris Cedex 13, France), AK(XMM-Newton Science Operations Centre, ESA, Villafranca del Castillo, Apartado 78, 28691 Villanueva de la Canada, Spain), AL(Max-Plank-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany), AM(Max-Plank-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany), AN(Max-Plank-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany), AO(Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK), AP(CEA, IRFU, Service d'Astrophysique, 91191 Gif-sur-Yvette, France ; Astroparticule et Cosmologie, 10 rue Alice Domont et Léonie Duquet, 75205 Paris Cedex 13, France)
Publication:
Astronomy and Astrophysics, Volume 488, Issue 2, 2008, pp.549-557 (A&A Homepage)
Publication Date:
09/2008
Origin:
EDP Sciences
Keywords:
Galaxy: center, X-rays: individuals: Sgr A*, X-rays: general, radiation mechanisms: general
DOI:
10.1051/0004-6361:200809986
Bibliographic Code:
2008A&A...488..549P

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 NH = 12.3+2.1-1.8× 1022 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 (NH = 8.8+4.4-3.2 × 1022 cm-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 Sgr A* (AV ~ 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.

Appendices A-C are only available in electronic form at http://www.aanda.org

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