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F. K. Baganoff, M. W. Bautz, W. Cui, J. P. Doty, G. R. Ricker (MIT), W. N. Brandt, E. D. Feigelson, G. P. Garmire, Y. Maeda, L. K. Townsley (PSU), M. Morris (UCLA), S. H. Pravdo (JPL)
X-ray emission has been detected from Sgr~A* for the first time in a 50-ks {\em Chandra} observation with the ACIS-I instrument. The newly discovered X-ray source, CXO~J174540.0-290027, is coincident with the radio position of Sgr~A* to within 0\farcs35, corresponding to a maximum projected distance of 16~light-days for an assumed distance to the center of our Galaxy of 8.0~kpc. CXO~J174540.0-290027 is the third brightest X-ray source in the entire 17\arcmin \times 17\arcmin field of view and the second brightest heavily absorbed source. Since the brightest absorbed source lies \approx 0\farcm5 from Sgr~A*, the probability of detecting an absorbed source by random chance that is as bright or brighter than the Sgr~A* candidate and that lies within 0\farcs35 of the radio position is about 3 \times 10-4.
The best-fit absorbed power-law model has a photon index \Gamma \approx 2.7 (\rm N(E) \propto E-\Gamma) and a column density \rm NH \approx 1 \times 1023\ cm-2. The measured (absorbed) flux in the 2--10~keV passband is \rm \approx 1\times 10-13\ ergs\ cm-2\ s-1, and the absorption-corrected luminosity is \rm \approx 2 \times 1033\ ergs\ s-1. The spectrum is equally well fit by an absorbed thermal bremsstrahlung model with \rm kT \approx 4\ keV and \rm NH \approx 8 \times 1022\ cm-2. Implications of the measured flux and spectrum for ADAF, CDAF, jet-disk, and other emission models are discussed. Comparison of the X-ray spectral shape and LX/LR ratio of Sgr~A* to a sample of LLAGN indicates that their high-energy emission mechanisms are significantly different.
This research was supported by NASA grant NAS8-38252.
The author(s) of this abstract have provided an email address for comments about the abstract: fkb@space.mit.edu