The Relativistic Disk in Sgr A*

Siming Liu1, Fulvio Melia1,2 and Benjamin C. Bromley3

1 Department of Physics, University of Arizona, Tucson, AZ, 85721.
2 Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ, 85721.
3 Department of Physics, University of Utah, 201 James Fletcher Building, Salt Lake City, UT 84112.


See: ApJ, 555, L83 (2001) and ApJ, 573, L23 (2002)

The detection of a mm/sub-mm ``bump'' in Sgr A*'s radio spectrum suggests that at least a portion of its overall radiative emission is produced in a compact accretion disk. This inference is strengthened by the recent observation of strong linear polarization (at the 10 percent level) within this bump. No linear polarization has been detected yet at other wavelengths. Given that higher frequencies are produced on smaller spatial scales, the mm/sub-mm bump probably arises in the vicinity of the black hole. We have found that a small (30 Schwarzschild-radii) magnetized accretion disk can not only account for the spectral bump via thermal synchrotron processes, but that it can also reproduce the corresponding polarimetric results. In connection with this, the quiescent X-ray emission appears to be associated with synchrotron self-Comptonization. In this talk, we will discuss the expected polarimetric imaging of Sgr A* that future mm-VLBI may achieve, and how these observations will help us probe the spacetime within a mere handful of Schwarzschild radii of Sgr A*'s event horizon. Other evidence for a disk of this kind around Sgr A* may also have been provided by the identification of a 106-day cycle in its radio variability. This behavior is intriguing because it may signal a precession of the disk around a spinning black hole. The disk's characteristics indicate rigid-body rotation, so the long precession period then suggests a small black hole spin with a spin parameter a/M of about 0.1. Such a small value would be favored if the nonthermal portion of Sgr A*'s spectrum is powered by a Blandford-Znajek type of process, for which the observed luminosity would correspond to an outer disk radius of about 30 Schwarzschild radii. This structure is consistent with earlier hydrodynamical simulations and is implied by Sgr A*'s mm/sub-mm spectral and polarimetric characteristics.