Coordinated NIR/mm observations of flare emission from Sagittarius A*

¹ I.Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany e-mail: devaky@ph1.uni-koeln.de
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Instituto de Astrofísica de Andalucía (CSIC), Camino Bajo de Huétor 50, 18008 Granada, Spain
⁴ MKI, 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 
Institut de Radio Astronomie Millimetrique, Domaine Universitaire, 38406 Saint Martin d'Heres, France
¹⁰ IPAC, California Institute of Technology, 770 South Wilson Avenue, Pasadena, CA 91125, USA
¹¹ LATT, Université de Toulouse, CNRS, 14 Avenue Edouard Belin, 31400 Toulouse, France
¹² Departamento de Astrofísica, Centro de Astrobiología, CSIC-INTA, Ctra. Torrejón a Ajalvir km 4, 28850 Torrejń de Ardoz, Spain
¹³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁴ National Radio Astronomy Observatory, PO Box 0, Socorro, NM 87801, USA
¹⁵ Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
¹⁶ IRAM, Avenida Divina Pastora 7, Núcleo Central, 18012 Granada, Spain

Received: 5 November 2009
Accepted: 29 March 2010

Abstract

Context. We report on a successful, simultaneous observation and modelling of the millimeter (mm) to near-infrared (NIR) flare emission of the Sgr A* counterpart associated with the supermassive (4 × 10⁶ ) black hole at the Galactic centre (GC). We present a mm/sub-mm light curve of Sgr A* with one of the highest quality continuous time coverages.

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

Methods. Our non-relativistic modelling is based on simultaneous observations carried out in May 2007 and 2008, using the NACO adaptive optics (AO) instrument at the ESO's VLT and the mm telescope arrays CARMA in California, ATCA in Australia, and the 30 m IRAM telescope in Spain. We emphasize the importance of multi-wavelength simultaneous fitting as a tool for imposing adequate constraints on the flare modelling. We present a new method for obtaining concatenated light curves of the compact mm-source Sgr A* from single dish telescopes and interferometers in the presence of significant flux density contributions from an extended and only partially resolved source.

Results. The observations detect flaring activity in both the mm domain and the NIR. Inspection and modelling of the light curves show that in the case of the flare event on 17 May 2007, the mm emission follows the NIR flare emission with a delay of 1.5±0.5 h. On 15 May 2007, the NIR flare emission is also followed by elevated mm-emission. We explain the flare emission delay by an adiabatic expansion of source components. For two other NIR flares, we can only provide an upper limit to any accompanying mm-emission of about 0.2 Jy. The derived physical quantities that describe the flare emission give a source component expansion speed of v_exp ~ 0.005c–0.017c, source sizes of about one Schwarzschild radius, flux densities of a few Janskys, and spectral indices of α = 0.6 to 1.3. These source components peak in the THz regime.

Conclusions. These parameters suggest that either the adiabatically expanding source components 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*. Applying the flux density values or limits in the mm- and X-ray domain to the observed flare events constrains the turnover frequency of the synchrotron components that are on average not lower than about 1 THz, such that the optically thick peak flux densities at or below these turnover frequencies do not exceed, on average, about ~1 Jy.