Limits on the Short Term Variability of Sagittarius A* in the Near-IR

S.D. Hornstein1, A.M. Ghez1,2, A. Tanner1, M. Morris1, E.E. Becklin1, P. Wizinowich3

1 Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1562
2 Institute for Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1565
3 W.M. Keck Observatory, 65-1120 Mamalahoa Hwy., Kamuela, HI 96743

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ApJL, submitted (2002)

The recent detection of a 3-hr X-ray flare by the Chandra Observatory has raised the possibility of enhanced emission over a broad range of wavelengths from Sgr A*, the suspected 2.6 x 106 M black hole at the Galactic Center, during a flaring event. We have, therefore, reconstructed 3-hr data sets from 2um speckle and adaptive optics images ( = 50 - 110 mas) obtained with the W. M. Keck 10-m telescopes between 1995 and 2001. In 25 separate observations, no evidence of any significant excess emission associated with Sgr A* was detected. The lowest of our detection limits gives an observed limit for the quiescent state of Sgr A* of 0.090.005 mJy, or, equivalently, a dereddened value of 2.00.1 mJy, which is a factor of 2 lower than the best previously published quiescent value. Under the assumption that there are random 3-hr flares producing both enhanced X-ray and near-infrared emission, our highest limit constrains the variable state of Sgr A* to <0.8 mJy (observed) or 19 mJy (dereddened). These results suggest that the model favored by Markoff et al. (2001), in which the flare is produced through local heating of relativistic particles surrounding Sgr A* (e.g., a sudden magnetic reconnection event), is unlikely, because it predicts peak 2um emission of 300 mJy, well above our detection limit.