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A dual emission mechanism at the Galactic Centre ?

1er juillet 2005 A dual emission mechanism at the Galactic Centre ?

Thanks to recent Galactic Centre observations performed in the thermal infrared with NACO at VLT, astronomers working at LESIA (Observatoire de Paris) and at ESO have found a correlation between the position and the flux of the 3.8 microns counterpart of Sgr A*, the bright radio source associated to the central black hole. This correlation results from the combined emission of the source of the flares recently discovered in the near-infrared at 2.2 microns and an extended dust structure, named Sgr A*-f. The 3.8 microns emission from Sgr A* would then associate sudden energetic events in the close environment of the black hole and a quiescent emission from Sgr A*-f heated by a colliding Sgr A* jet.

Since late 2001 and the installation of the adaptive optics system NAOS and its camera CONICA, the core of our own Galaxy has revealed many of its hidden secrets. There was firstly the confirmation of a black hole lying at the centre of the Galaxy, associated to the bright radio source Sgr A* and then the detection in the near-infrared (K-band at 2.2 microns) of sudden bright flares from hot gas falling into the black hole

During the 2004 campaign of the Galactic Centre observations, analogous flares have been observed in the thermal infrared at 3.8 microns (cf. Figure 1). The flares are similarly evolving on short timescales, about few tens of minutes, with a flux amplification factor about 2 to 3. But the astronomers from LESIA and ESO have additionally observed a correlation between the photocentre position and the flux of the 3.8 mm counterpart of Sgr A* (Sgr A*/L’), pointing at a dual emission mechanism from this counterpart (cf. Figure 2) :- the flaring source, point-like and associated to the Sgr A* counterpart itself (Sgr A*/IR) ;- a quiescent source, resolved with an extension of about 130 milli-arcseconds, located at 75 milli-arcseconds (600 astronomical units) from the latter and named Sgr A*-f.

Figure 2 : Position du photocentre de Sgr A*/L’ en fonction de sa magnitude à 3,8 microns. Chaque symbole correspond à une des 6 nuits d’observations.

The emission from Sgr A*-f cannot be stellar since it is extended, with a too large infrared colour index. It cannot be either explained by dust heated in the accretion disk by Sgr A* since the observed colour temperature is one order of magnitude larger than the equilibrium temperature that dust grains would reach at this distance from Sgr A*. A mechanism explaining the Sgr A*-f luminosity could be the energy transfer from a Sgr A* jet to the dusty material surrounding Sgr A* through collisions : the kinetic power linked to the jet mass loss is comparable to the Sgr A*-f luminosity, from 10 to 30 Lsun. Future high angular resolution observations covering simultaneously the near and the thermal infrared are already scheduled with NACO in order to confirm this dual emission mechanism and to constrain the physical processes giving rise to it.

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