Letter to the Editor

Detection of the Sgr A* activity at 3.8 and 4.8 m with NACO^*

¹ European Southern Observatory (ESO), Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany e-mail: yclenet@eso.org
² Observatoire de Paris, LESIA, 5 place Jules Janssen, 92195 Paris Cedex, France e-mail: [daniel.rouan; damien.gratadour; francois.lacombe; eric.gendron; pierre.lena]@obspm.fr
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching bei München, Germany e-mail: [genzel; ott]@mpe.mpg.de
⁴ I. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937, Köln, Germany e-mail: rainer@ph1.uni-koeln.de

Received: 27 April 2004
Accepted: 25 July 2004

Abstract

L'-band ( 3.8 μm) and M'-band ( 4.8 μm) observations of the Galactic Center region, performed in 2003 at VLT (ESO) with the adaptive optics imager NACO, have lead to the detection of an infrared counterpart of the radio source Sgr A* at both wavelengths. The measured fluxes confirm that the Sgr A* infrared spectrum is dominated by the synchrotron emission of nonthermal electrons. The infrared counterpart exhibits no significant short term variability but demonstrates flux variations on daily and yearly scales. The observed emission arises away from the position of the dynamical center of the S2 orbit and would then not originate from the closest regions of the black hole.