Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1988
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988mnras.233..489a&link_type=abstract
Monthly Notices of the Royal Astronomical Society (ISSN 0035-8711), vol. 233, July 15, 1988, p. 489-502. CNPq-supported research
Astronomy and Astrophysics
Astronomy
17
Active Galactic Nuclei, Black Holes (Astronomy), Metric Space, Viscous Flow, Accretion Disks, Kerr Effects, Schwarzschild Metric, Shear Layers
Scientific paper
The inaccuracy of models of viscous flows on to black holes which assume a zero-torque boundary condition at the radius of the last stable Keplerian orbit is examined. It is shown that the existence of a nonzero viscous torque at the horizon of a black hole does not require the transfer of energy or angular momentum across any spacelike distance, and therefore does not violate causality. It is suggested that in a range of cold flows outside slowly rotating Kerr holes, the viscous torque on the horizon is reversed. In the models considered, this effect can only occur if there is a reversal of the density gradient near the horizon so that the fluid is expanding as it falls inward. Despite such reversals in the viscous torque, it is found that the net torque on the hole is always directed in the normal sense, provided that the flow remains steady. A theorem relating the specific energy to the angular velocity in hot viscously driven flows is derived and applied to a simple disk model.
Anderson Malcolm R.
Lemos José P. S.
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