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Stellar dynamics in the innermost region

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Author Topic: Stellar dynamics in the innermost region  (Read 37 times)
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« on: August 04, 2007, 01:20:48 am »

Near-infrared flares from the black hole 

Near-infrared high-resolution observations of the galactic centre (GC) became possible since the beginning of the 1990s. Since then, the GC stellar was regularly monitored by high-resolution NIR imaging. However, in spite of all efforts, no unambiguous NIR counterpart of SgrA* could be detected up to 2003. On the 9th of May, during routine observations of the GC star cluster at 1.7 microns with NAOS/CONICA at the VLT, we witnessed a powerful flare at the location of the black hole. Within a few minutes, the flux of a faint source increased by a factor of 5-6 and fainted again after about 30 min. The flare was found to have happened within a few milli-arcseconds of the position of Sgr A*. The short rise-and-decay times told us that the source of the flare was located within less than 10 Schwarzschild radii of the black hole. During subsequent observations in 2003 and 2004, we could observe more flares from Sgr A* in the H, K and L-bands (1.7, 2.2 and 3.8 microns) and also quiescent emission from a source at this location. With hindsight, we could also detect a flaring source in older, longer wavelength data from 2002. Independently, flaring and variability of SgrA* in the L-band was also observed at the Keck telescope by researchers from the University of California, LA, in June 2003.

The quiescent and flaring NIR emission from Sgr A* fills an important gap in our knowledge of the spectrum of this source and will allow to constrain the existing models on how the radiation is produced. While the quiescent emission appears to be largely consistent with an origin in the high-energy tail of a synchrotron spectrum, the mechanism of the NIR flares is uncertain and its explanation one of our main goals. Simultaneous, multi-wavelength NIR and X-ray observations of the GC were executed in 2004. Additionally we observed Sgr A* with the AO-assisted near infrared integral field spectrometer SINFONI in July 2004 and were able to detect and measure a weak flare, providing the first spectrum of a Sgr A* flare ever obtained.

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