A polarized infrared flare from Sagittarius A* and the signatures of orbiting plasma hotspots

In this article we summarize and discuss the infrared, radio and X-ray emission from the supermassive black hole in the Galactic Centre, SgrA*. We include new results from near-infrared polarimetric imaging observations obtained on 2006 May 31. In that night, a strong flare in K_s band (2.08 μm) reaching top fluxes of ~16 mJy could be observed. This flare was highly polarized (up to ~40 per cent) and showed clear substructure on a time-scale of 15 min, including a swing in the polarization angle of about 70°. For the first time we were able to observe both polarized flux and short-time variability, with high significance in the same flare event. This result adds decisive information to the puzzle of the SgrA* activity. The observed polarization angle during the flare peak is the same as observed in two events in 2004 and 2005. Our observations strongly support the dynamical emission model of a decaying plasma hotspot orbiting SgrA* on a relativistic orbit. The observed polarization parameters and their variability with time might allow to constrain the orientation of accretion disc and spin axis with respect to the Galaxy.

Partially based on observations at the Very Large Telescope (VLT) of the European Southern Observatory (ESO), Chile.

E-mail: trippe@mpe.mpg.de ‡

Present address: Observatoire de Paris - Section de Meudon, 5 Place Jules Janssen, F-92195 Meudon Cedex, France.