Flares and variability from Sagittarius A*: five nights of simultaneous multi-wavelength observations^⋆

¹ LESIA-Observatoire de Paris, CNRS UMR 8109, UPMC Univ. Paris 6, Université Paris Diderot, 5 place Jules Janssen, 92195 Meudon, France
e-mail: xavier.haubois@obspm.fr
² MPE, Max-Planck-Institut für Extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
³ MPIfR, Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53321 Bonn, Germany
⁴ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária, São Paulo, 05508-900, Brazil

Received: 18 July 2011
Accepted: 11 January 2012

Abstract

Aims. We report on simultaneous observations and modeling of mid-infrared (MIR), near-infrared (NIR), and submillimeter (sub-mm) emission of the source Sgr A* associated with the supermassive black hole at the center of our Galaxy. Our goal was to monitor the activity of Sgr A* at different wavelengths in order to constrain the emitting processes and gain insight into the nature of the close environment of Sgr A*.

Methods. We used the MIR instrument VISIR in the BURST imaging mode, the adaptive optics assisted NIR camera NACO, and the sub-mm antenna APEX to monitor Sgr A* over several nights in July 2007.

Results. The observations reveal remarkable variability in the NIR and sub-mm during the five nights of observation. No source was detected in the MIR, but we derived the lowest upper limit for a flare at 8.59 μm (22.4 mJy with A_{8.59 μm} = 1.6 ± 0.5). This observational constraint makes us discard the observed NIR emission as coming from a thermal component emitting at sub-mm frequencies. Moreover, comparison of the sub-mm and NIR variability shows that the highest NIR fluxes (flares) are coincident with the lowest sub-mm levels of our five-night campaign involving three flares. We explain this behavior by a loss of electrons to the system and/or by a decrease in the magnetic field, as might conceivably occur in scenarios involving fast outflows and/or magnetic reconnection.