Polarized NIR and X-ray flares from Sagittarius A*

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str.77, 50937 Köln, Germany e-mail: eckart@ph1.uni-koeln.de
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA e-mail: fkb@space.mit.edu
⁴ Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, CA 90095-1562, USA
⁵ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802-6305, USA
⁶ Institut für Theoretische Physik und Astrophysik Christian-Albrechts-Universität zu Kiel Leibnizstr. 15 24118 Kiel, Germany
⁷ Astronomical Institute, Academy of Sciences, Boční II, 14131 Prague, Czech Republic
⁸ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098SJ Amsterdam, The Netherlands
⁹ Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Científicas, CSIC, Serrano 121, 28006 Madrid, Spain
¹⁰ Observatoire Midi-Pyrénées, 14 avenue Édouard Belin, 31400 Toulouse, France

Received: 4 October 2007
Accepted: 6 December 2007

Abstract

Context.Stellar dynamics indicate the presence of a supermassive 3-4  10⁶  black hole at the Galactic Center. It is associated with the variable radio, near-infrared, and X-ray source Sagittarius A* (SgrA*).

Aims.The goal is the investigation and understanding of the physical processes responsible for the variable emission from SgrA*.

Methods.The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope (July 2005, May 2007) and the ACIS-I instrument aboard the Chandra X-ray Observatory (July 2005).

Results.We find that for the July 2005 flare the variable and polarized NIR emission of SgrA* occurred synchronous with a moderately bright flare event in the X-ray domain with an excess 2-8 keV luminosity of about 8  10³³ erg/s. We find no time lag between the flare events in the two wavelength bands with a lower limit of ≤10 min. The May 2007 flare shows the highest sub-flare to flare contrast observed until now. It provides evidence for a variation in the profile of consecutive sub-flares.

Conclusions.We confirm that highly variable and NIR polarized flare emission is non-thermal and that there exists a class of synchronous NIR/X-ray flares. We find that the flaring state can be explained via the synchrotron self-Compton (SSC) process involving up-scattered X-rays from the compact source component. The observations can be interpreted in a model involving a temporary disk with a short jet. In the disk component the flux density variations can be explained by spots on relativistic orbits around the central supermassive black hole (SMBH). The profile variations for the May 2007 flare can be interpreted as a variation of the spot structure due to differential rotation within the disk.