The flare activity of Sagittarius A*

New coordinated mm to X-ray observations

¹ I.Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany e-mail: eckart@ph1.uni-koeln.de
² Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA e-mail: fkb@space.mit.edu
³ Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, CA 90095-1562, USA e-mail: morris@astro.ucla.edu
⁴ Max Planck Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
⁵ Department of Astronomy and Radio Astronomy Laboratory, University of California at Berkeley, Le Conte Hall, Berkeley, CA 94720, USA
⁶ Department of Astronomy and Radio Astronomy Laboratory, University of California at Berkeley, Campbell Hall, Berkeley, CA 94720, USA e-mail: gbower@astro.berkeley.edu
⁷ Harvard-Smithsonian Center for Astrophysics, Cambridge MA 02138, USA e-mail: dmarrone.cfa.harvard.edu
⁸ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802-6305, USA
⁹ Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA

Received: 26 October 2005
Accepted: 14 December 2005

Abstract

Context.We report new simultaneous near-infrared/sub-millimeter/X-ray observations of the Sgr A* counterpart associated with the massive   black hole at the Galactic Center.

Aims. We investigate the physical processes responsible for the variable emission from Sgr A*.

Methods. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope and the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the Submillimeter Array SMA on Mauna Kea, Hawaii, and the Very Large Array in New Mexico.

Results. We detected one moderately bright flare event in the X-ray domain and 5 events at infrared wavelengths. The X-ray flare had an excess 2-8 keV luminosity of about  erg/s. The duration of this flare was completely covered in the infrared and it was detected as a simultaneous NIR event with a time lag of ≤10 min. Simultaneous infrared/X-ray observations are available for 4 flares. All simultaneously covered flares, combined with the flare covered in 2003, indicate that the time-lag between the NIR and X-ray flare emission is very small and in agreement with a synchronous evolution. There are no simultaneous flare detections between the NIR/X-ray data and the VLA and SMA data. The excess flux densities detected in the radio and sub-millimeter domain may be linked with the flare activity observed at shorter wavelengths.

Conclusions. We find that the flaring state can be explained with a synchrotron self-Compton (SSC) model involving up-scattered sub-millimeter photons from a compact source component. This model allows for NIR flux density contributions from both the synchrotron and SSC mechanisms. Indications for an exponential cutoff of the NIR/MIR synchrotron spectrum allow for a straightforward explanation of the variable and red spectral indices of NIR flares.