The Intrinsic Variability of Sgr A* at Radio Wavelengths

Jun-Hui Zhao

Harvard-Smithsonian Center for Astrophysics


We will review and discuss the variability of Sgr A* based on the analysis of the light curves obtained from the monitoring programs that have been carried out with both the VLA at the wavelengths around 1 cm and the partially completed SMA at 1 mm.

With the densely sampled light curves observed with the VLA over the past two years, we have confirmed the previous finding of the periodic signals with a period of 100 d from the VLA archival data. We have also found an additional periodic signal with a period of 2.4-2.5 times longer than the short period. Our observations also suggest that both the periods of the individual oscillations drift in time while the ratio of the periods appears to remain in a constant, suggesting a double quasi-periodic oscillation (DQPO) in flux density of Sgr A*. The DQPO of Sgr A* suggests that a dynamic system consisting of a supermassive black hole, an accretion flow, and an orbiting perturber is subject to an orbital resonance which may periodically trigger the central activities. (Credit: Jun-Hui Zhao, J. Pegg, R. M. Herrnstein, G. C. Bower, & W. M. Goss, 2002).

With the partially finished Sub-millimeter Array on Mauna Kea, a total of 25 epochs of observations were carried out over the past 15 months. Noticeable variations in flux density at 1.3 mm were observed showing three ``flares''. The SMA observations suggest that Sgr A* is highly peaked towards sub-millimeter wavelengths during a flare suggesting the presence of a break in the spectrum at wavelength around 3 mm. A cross-correlation of the SMA data at 1 mm with the data obtained with the VLA at 1 cm shows a global delay of two weeks, suggesting that sub-millimeter wavelengths tend to peak first. The cross-correlation function also suggests that a periodic oscillation at a period of 1345 d is present in both the light curves observed with the VLA and the SMA at 1.3 cm and 1.3 mm, respectively. Both the VLA and the SMA observations suggest that Sgr A* might have been regularly powered. (Credit:Jun-Hui Zhao, K. Y. Young, R. S. Herrnstein, P. T. P. Ho, T. Tsutsumi, K. Y. Lo, W. M. Goss, G. C. Bower, 2002).