Modeling mm- to X-ray flare emission 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 e-mail: morris@astro.ucla.edu
⁵ Instituto de Astrofísica de Andalucía, CSIC, Camino Bajo de Huétor 50, 18008 Granada, Spain e-mail: rainer@iaa.es
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: 16 November 2008
Accepted: 17 March 2009

Abstract

Context. We report on new modeling results based on the mm- to X-ray emission of the SgrA* counterpart associated with the massive ~410⁶  black hole at the Galactic Center.

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

Methods. Our modeling is based on simultaneous observations carried out on 07 July, 2004, 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. The observations revealed several flare events in all wavelength domains. Here we show that the flare emission can be described with a combination of a synchrotron self-Compton (SSC) model followed by an adiabatic expansion of the source components. The SSC emission at NIR and X-ray wavelengths involves up-scattered sub-millimeter photons from a compact source component. At the start of the flare, spectra of these components peak at frequencies between several 100 GHz and 2 THz. The adiabatic expansion then accounts for the variable emission observed at sub-mm/mm wavelengths. The derived physical quantities that describe the flare emission give a blob expansion speed of v_exp ~ 0.005 c, magnetic field of B around 60 G or less and spectral indices of α = 0.8 to 1.4, corresponding to a particle spectral index p ~ 2.6 to 3.8.

Conclusions. A combined SSC and adiabatic expansion model can fully account for the observed flare flux densities and delay times covering the spectral range from the X-ray to the mm-radio domain. The derived model parameters suggest that the adiabatic expansion takes place in source components that have a bulk motion larger than v_exp or the expanding material contributes to a corona or disk, confined to the immediate surroundings of SgrA*.