2002/05/28 by Siming Liu, Fulvio Melia
Physics and Astronomy · #Accretion (finance) #Astrophysical Phenomena and Observations #Black hole (networking) #Galactic Center #Intermediate-mass black hole #Precession #Pulsars and Gravitational Waves Research #Relativity and Gravitational Theory #Schwarzschild radius #Stellar black hole #Supermassive black hole #astro-ph
paper · pdf · doi:10.1086/341991
published as Astrophys.J. 573 (2002) L23 · 12 pages, 1 figure. Accepted for publicaton in ApJ Letters
arxiv created 2002/05/28 · openalex publication_date 2002/07/01 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/06
Sagittarius A* is a compact radio source at the Galactic center that is thought to be the radiative manifestation of a 2.6 × 10 6 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 < 10 r S ) 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, r S , for an object of this mass M is 2 GM / c 2 ≈ 7.7 × 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 ~30 r S , for which a ≈ ( M /10)( r o /30 r 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 (~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 r o ~ 30 r S . Such a small disk size is also suggested by earlier hydrodynamical simulations and is implied by Sgr A*'s spectral and polarimetric characteristics.