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S. D. Hornstein, A. M. Ghez (UCLA), K. Matthews (Caltech), J. R. Lu, M. Rafelski, M. Morris, E. E. Becklin (UCLA), D. Thompson (Caltech)
Using the newly commisioned Laser Guide Star Adaptive Optics (LGS-AO) system and the facility near-infrared camera (NIRC2) on the W. M. Keck II telescope, we have obtained a time series of diffraction-limited, high signal-to-noise ratio images of the near-infrared emission associated with the supermassive black hole at the Galactic center (SgrA*-IR). Over the course of 120 minutes, we imaged SgrA*-IR in the broad-bandpass filters H[1.6 \mum], K'[2.1 \mum], and L'[3.8 \mum] every 3 minutes, which allowed us to investigate the time-dependent and flux-dependent properties of SgrA*-IR. During these observations, Sgr A*-IR showed continuous flux density variations, ranging from 2 to 12 mJy (dereddened) in the K'-band images, without achieving any steady state level during the entire period of observations. Additional L'- and Ms[4.7 \mum]-band images were obtained on a separate night using NIRC2 with the Natural Guide Star AO system. Using these data sets to investigate the near-infrared spectral slope, \alpha, where F\nu \propto \nu\alpha, of the emission arriving from SgrA*-IR, we find no significant change in the spectral slope during any of the observations in which HSgr A*-IR or K'Sgr A*-IR \ge 5 mJy; we find \alpha = -0.7 ± 0.2, -0.4 ± 0.1, & -0.5 ± 0.7 for the H-K', K'-L', and L'-Ms light curves, respectively. Below 5 mJy, background emission, most likely from the underlying stellar population, contaminates the photometry of Sgr A*-IR at the shorter wavelengths (as SgrA*-IR is much redder than the stellar emission). These results imply that a) there is no spectral break from 1.6 - 4.6 \mum and b) the spectral slope is independent of the strength of Sgr A*-IR's emission from K=5-12 mJy. These results, coupled with the lack of steady state emission as well as the lack of any periodicity, support the hypothesis that all of the emission seen at IR wavelengths is due to synchrotron emission from a stochastically injected population of high-energy, non-thermal electrons.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.