Concurrent X-ray, near-infrared, sub-millimeter, and GeV gamma-ray observations of Sagittarius A*

¹ Service d’Astrophysique (SAp), IRFU, DSM, CEA-Saclay, 91191 Gif-sur-Yvette, France
e-mail: trap@apc.univ-paris7.fr
² AstroParticule & Cosmologie (APC), Université Paris VII, CNRS, CEA, Observatoire de Paris, 75205 Paris, France
³ Département de Physique, Palais de la découverte, Universcience, 75008 Paris, France
⁴ Max-Planck-Institut für Extraterrestrische Physik (MPE), 85748 Garching, Germany
⁵ Max-Planck-Institut für Radioastronomie (MPIfR), 53121 Bonn, Germany
⁶ School of Physics and Astronomy, University of Southhampton, Highfield, Southhampton, SO17 1BJ, UK
⁷ European Space Agency (ESA), ESAC, PO Box 778, Villanueva de la Canada, 28691 Madrid, Spain
⁸ Laboratoire d’Études Spatiales et d’Instrumentation en Astrophysique (LESIA), Observatoire de Paris, 92195 Meudon, France
⁹ Department of Physics and Steward Observatory, University of Arizona, Tucson, Arizona 85721, USA
¹⁰ Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA

Received: 4 June 2010
Accepted: 5 February 2011

Abstract

Aims. The radiative counterpart of the supermassive black hole at the Galactic center (GC), Sgr A^⋆, is subject to frequent flares that are visible simultaneously in X-rays and the near-infrared (NIR). Often, enhanced radio variability from centimeter to sub-millimeter wavelengths is observed to follow these X-ray/NIR eruptions. We present here a multi-wavelength campaign carried out in April 2009, with the aim of characterizing this broadband flaring activity.

Methods. Concurrent data from the XMM-Newton/EPIC (2–10 keV), VLT/NACO (2.1 μm, 3.8 μm), APEX/LABOCA (870 μm), and Fermi/LAT (0.1–200 GeV) instruments are employed to derive light curves and spectral energy distributions of new flares from Sgr A^⋆.

Results. We detected two relatively bright NIR flares, both associated with weak X-ray activity, one of which was followed by a strong sub-mm outburst  ~200 min later. Photometric spectral information on a NIR flare was obtained for the first time with NACO, giving a power-law photon index α =  −0.4 ± 0.3 (F_ν ∝ ν ^α). The first attempt to detect flaring activity from the Fermi GC source 1FGL J1745.6–2900 is also reported. We model NIR, X-ray, and sub-mm flares in the context of non-thermal emission processes. We find that the simplest scenario involving a single expanding plasmoid releasing synchrotron NIR/sub-mm and synchrotron self-Compton X-ray radiation is inadequate to reproduce the data, but we offer suggestions to reconcile the basic elements of the theory and the observations.