Spin-induced disk precession in the supermassive black hole at the Galactic center

Sagittarius A* is a compact radio source at the Galactic center that is thought to be the radiative manifestation of a 2.6 x 10(6) M-circle dot supermassive black hole. At least a portion of its spectrum-notably the millimeter/submillimeter "bump"-appears to be produced within the inner portion (r < 10r(S)) of a hot, magnetized Keplerian flow, whose characteristics are also consistent with the similar to 10% linear polarization detected from this source at millimeter wavelengths. (The Schwarzschild radius, r(S), for an object of this mass M is 2GM/c(2) approximate to 7.7 x 10(11) cm, or roughly 1/20 AU.) The recent detection of a 106 day cycle in Sgr A*'s radio variability adds significant intrigue to this picture since it may signal a precession of the disk induced by the spin a of the black hole. The dynamical timescale near the marginally stable orbit around an object with this mass is 20 minutes. Thus, since the physical conditions associated with the disk around Sgr A* imply rigid-body rotation, a precession period of 106 days may be indicative of a small black hole spin if the circularized flow is confined to a region similar to 30r(S), for which. a approximate to (M/10)(r(o)/30r(S))(5/2). The precession of a larger structure would require a bigger black hole spin. We note that a small value of a/M (<0.1) would be favored if the nonthermal (similar to1-20 cm) portion of Sgr A* 's spectrum is powered with energy extracted via a Blandford-Znajek type of process, for which the observed luminosity would correspond to an outer disk radius r(o) similar to 30r(S). Such a small disk size is also suggested by earlier hydrodynamical simulations and is implied by Sgr A*'s spectral and polarimetric characteristics.