Viscous Stability of Relativistic Keplerian Accretion Disks

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Abstract
We investigate the viscous stability of thin, Keplerian accretion disks in regions in which general relativistic (GR) effects are essential. For gas pressure-dominated (GPD) disks, we show that the Newtonian conclusion that such disks are viscously stable is reversed by GR modifications in the behaviors of viscous stress and surface density over a significantly large annular region not far from the innermost stable orbit at r=rms. For slowly rotating central objects, this region spans a range of radii 14 r 19 in units of the central object's mass M. For radiation pressure-dominated (RPD) disks, the Newtonian conclusion that they are viscously unstable remains valid after including the above GR modifications, except in a very small annulus around r ≈ 14 M, which has a negligible influence. Inclusion of the stabilizing effect of the mass inflow through the disk's inner edge via a GR analog of Roche lobe overflow adds a small, stable region around rms for RPD disks but leaves GPD disks unchanged. We mention possible astrophysical relevance of these results, particularly to the high-frequency X-ray variabilities observed by the Rossi X-Ray Timing Explorer.
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