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Aschenbach effect: unexpected topology changes in the motion of particles and fluids orbiting rapidly rotating Kerr black holes

Journal article
Authors Zdenek Stuchlík
Petr Slany
Gabriel Török
Marek A Abramowicz
Published in Phys. Rev. D
Volume 71
Pages 024037
ISSN 0556-2821
Publication year 2005
Published at Department of Physics (GU)
Pages 024037
Language en
Subject categories Physical Sciences, Astronomy, Astrophysics and Cosmology


Newtonian theory predicts that the velocity [script V] of free test particles on circular orbits around a spherical gravity center is a decreasing function of the orbital radius r, d[script V]/dr<0. Only very recently, Aschenbach [B. Aschenbach, Astronomy and Astrophysics, 425, 1075 (2004)] has shown that, unexpectedly, the same is not true for particles orbiting black holes: for Kerr black holes with the spin parameter a>0.9953, the velocity has a positive radial gradient for geodesic, stable, circular orbits in a small radial range close to the black-hole horizon. We show here that the Aschenbach effect occurs also for nongeodesic circular orbits with constant specific angular momentum [script-l] = [script-l]0 = const. In Newtonian theory it is [script V] = [script-l]0/[script R], with [script R] being the cylindrical radius. The equivelocity surfaces coincide with the [script R] = const surfaces which, of course, are just coaxial cylinders. It was previously known that in the black-hole case this simple topology changes because one of the "cylinders" self-crosses. The results indicate that the Aschenbach effect is connected to a second topology change that for the [script-l] = const tori occurs only for very highly spinning black holes, a>0.99979.

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