Simultaneous NIR/sub-mm observation of 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 Instituto de Astrofísica de Andalucía, Camino Bajo de Huétor 50, 18008 Granada, Spain
⁴ Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA
⁵ Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USA
⁶ Astronomical Institute, Academy of Sciences, Boční II, 14131 Prague, Czech Republic
⁷ Institut für Theoretische Physik und Astrophysik, Christian-Albrechts-Universität zu Kiel, Leibnizstr. 15, 24118 Kiel, Germany
⁸ Steward Observatory, The University of Arizona, 933 N. Cherry Ave. Tucson, AZ 85721, USA
⁹ Harvard-Smithsonian Center for Astrophysics, SMA project, 60 Garden Street, MS 78 Cambridge, MA 02138, USA
¹⁰ Institut de Radio Astronomie Millimetrique, Domaine Universitaire, 38406 Saint-Martin d'Hères, France
¹¹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098SJ Amsterdam, The Netherlands
¹² LATT, Université de Toulouse, CNRS, 14 avenue Édouard Belin, 31400 Toulouse, France
¹³ DAMIR, Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Científicas, Serrano 121, 28006 Madrid, Spain
¹⁴ W.M. Keck Observatory (WMKO), CARA, 65-1120 Mamalahoa Hwy., Kamuela, HI-96743, USA
¹⁵ National Radio Astronomy Observatory, PO Box 0, Socorro, NM 87801, USA
¹⁶ Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
¹⁷ IRAM, Avenida Divina Pastora, 7, Núcleo Central, 18012 Granada, Spain

Received: 6 September 2008
Accepted: 23 October 2008

Abstract

Context. We report on a successful, simultaneous observation and modeling of the sub-millimeter to near-infrared flare emission of the Sgr A* counterpart associated with the super-massive (4 10⁶  ) black hole at the Galactic center.

Aims. 
We study and model the physical processes giving rise to the variable emission of Sgr A*.

Methods. Our non-relativistic modeling is based on simultaneous observations that have been carried out on 03 June, 2008. We used the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope and the LABOCA bolometer at the Atacama Pathfinder Experiment (APEX). We emphasize the importance of a multi-wavelength simultaneous fitting as a tool for imposing adequate constraints on the flare modeling.

Results. The observations reveal strong flare activity in the 0.87 mm (345 GHz) sub-mm domain and in the 3.8 μ/2.2 μm NIR. Inspection and modeling of the light curves show that the sub-mm follows the NIR emission with a delay of 1.5 ± 0.5 h. We explain the flare emission delay by an adiabatic expansion of the source components. The derived physical quantities that describe the flare emission give a source component expansion speed of v_exp ~ 0.005c, source sizes around one Schwarzschild radius with flux densities of a few Janskys, and spectral indices of α = 0.8 to 1.8, corresponding to particle spectral indices ~2.6 to 4.6. At the start of the flare the spectra of these components peak at frequencies of a few THz.

Conclusions. These parameters suggest that the adiabatically expanding source components either have a bulk motion greater than v_exp or the expanding material contributes to a corona or disk, confined to the immediate surroundings of Sgr A*.