[Previous] | [Session 103] | [Next]
K.H. Young, Jun-Hui Zhao, R.S. McGary, P.T.P. Ho, T. Tsutsumi (CfA), W. M. Goss (NRAO), G. C. Bower (UC Berkeley), SMA Team
We observed Sgr A* at 1.3 and 0.87 mm using the partially completed Submillimeter Array (SMA) with 3 to 4 antennas ranging the baselines from 10 to 60 k\lambda at 1.3 mm. For baselines \ge 20 k\lambda which were used in the measurements, the confusing flux density at 1.3 mm from the surrounding extended emission is \le 300 mJy, which was taken out in the UV domain. The typical uncertainty of 10%--20% is dominated by the residual systematic errors, e.g. a poor telescope pointing model. The SMA light curve, derived from 16 epoch observations at 1.3 mm over the past seven months from March to September, 2001, shows two strong flares from SgrA*. The March flare (started from 4.1±0.5 Jy after an unseen peak and decreased to 1.1±.15 Jy in three months) appeared to be relatively stronger than that of July (3.1±0.3 Jy in peak), showing a good correlation between the 1.3 mm flux densities and those at 1.3 cm observed from a VLA monitoring program. Considering the fact of the unseen peak, there might be a time delay between the wavelengths of 1.3 mm and 1.3 cm during the March event. During the course of the second event the 1.3-cm flux density increased slowly, showing a significant delay ~7 weeks in reaching a peak as compared to the 1.3 mm light curve. In addition, the spectral index (\propto\nu\alpha) varied from \alpha =0.5±.1 of the March flare to ~.23±.07 in the June minimum and then increased to \alpha =0.6±.1 in the July flare. The time delay and the spectral index variation are consistent with a picture that the events arise from the inner part of SgrA* due to some energy releases from the accretion process around the supermassive black hole and SgrA* is wrapped up in a thick plasma consisting of different layers of characteristic surfaces. Our observations suggest that SgrA* is highly variable at the wavelengths ranging from short millimeter to submillimeter.