Spin-induced Disk Precession in the Supermassive Black Hole at the Galactic Center

Abstract

Sagittarius A* is a compact radio source at the Galactic center that is thought to be the radiative manifestation of a 2.6 × 106 M☉ supermassive black hole. At least a portion of its spectrum—notably the millimeter/submillimeter "bump"—appears to be produced within the inner portion (r < 10rS) of a hot, magnetized Keplerian flow, whose characteristics are also consistent with the ~10% linear polarization detected from this source at millimeter wavelengths. (The Schwarzschild radius, rS, for an object of this mass M is 2GM/c2 ≈ 7.7 × 1011 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 ~30rS, for which a ≈ (M/10)(ro/30rS)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 (~1-20 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 ro ~ 30rS. Such a small disk size is also suggested by earlier hydrodynamical simulations and is implied by Sgr A*'s spectral and polarimetric characteristics.