Monitoring Sgr A* with the VLA

The bright radio point source, Sgr A*, is thought to be the result of emission from material very close to a supermassive black hole at the center of the Milky Way Galaxy. This supermassive black hole has a mass more than 4 million times that of our Sun! Although more massive black holes have been detected in the centers of other Galaxies, the center of the Milky Way is special because it is so nearby (relatively speaking!). At a distance of only 8000 parsecs (24,000 light years), the center of our Galaxy offers a unique opportunity to study the environment around a supermassive black hole in great detail.

Previous observations have indicated that the varibility of Sgr A* may be periodic (Zhao et al 2001). However, the sampling of the data was insufficient to understand the varibility in great detail or search for short periodicities (< 50 days). A new, long-term monitoring program was needed... Since June 2000, we have monitored the flux density (brightness) of Sgr A* at the VLA at three radio wavelengths (U BAND (2.0 cm), K Band (1.3 cm), and Q BAND (0.7 cm)) in order to understand the variability of this source. Changes in the flux density of Sgr A* allow us to probe the physics of accretion onto the supermassive black hole.

From June 2000 to January 2003, observations were made roughly every 10 days, allowing us to look for periodicities as short as ~20 days. We have successfully ruled out periods shorter than ~100 days and now observe Sgr A* roughly once a month to continue to earch of long-period variability. By regularly observing Sgr A*, we hope to better constrain the periodicity of this source. In addition, we have triggered VLBA observations of Sgr A* when it is in its brightest state ("flare") to see if any morphological change in the source is observable.

For a more detailed (but slightly out-of-date) overview of our project check out this "mini" version of our poster
from the January 2002 AAS (PDF).
We have also monitored the flux of Sgr A* at 1.3~mm with the new Sub-Millimeter Array: "Variability of Sgr A* -- Flares at 1 Millimeter," Zhao, J.-H., Young, K.H, Herrnstein, R.M., Ho, P.T.P., Tsutsumi, T., Lo, K.Y., Bower, G.C., & Goss, W.M. 2003, ApJL, 586, 29

Bower, G.C., Falcke, H., Herrnstein, R.M., Zhao, J.-H., & Goss, W.M., & Backer, D.C. "Detection of the Intrinsic Size of Sagittarius A* through Closure Amplitude Imaging," 2004, Science, 304, 704

Zhao, J.-H., Herrnstein, R.M., Bower, G.C., Goss, W.M, & Liu, S.M. " A Radio Outburst Coincident with the Large X-ray Flare from Sgr A* on 2002 October 3," 2004, ApJ Letters, 603, 85

Observing Schedule:
Observing Schedule for Upcoming Months:
Beginning in 2003, we observe once per month (see above ) as part of AZ143.

Observation Date Bands Array Status
59643 2003-10-14 U,K,Q BnA (18:30 - 20:30 LST) reduced
59643 2003-11-10 U,K,Q B (16:00 - 18:00 LST)
59643 2003-12-20 U,K,Q B (16:00 - 18:00 LST)
59643 2004-01-25 U,K,Q B (17:30 - 19:30 LST)
59765 2004-02-13 U,K,Q CnB (16:00 - 18:00 LST)
59795 2004-03-14 U,K,Q C (18:00 - 20:00 LST)