Radii of spherical timelike orbits around Kerr black holes

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PHYSICAL REVIEW D, vol.104, no.12, 2021 (SCI-Expanded) identifier identifier identifier

  • Publication Type: Article / Article
  • Volume: 104 Issue: 12
  • Publication Date: 2021
  • Doi Number: 10.1103/physrevd.104.124059
  • Journal Name: PHYSICAL REVIEW D
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus, Academic Search Premier, Aerospace Database, Chemical Abstracts Core, INSPEC, zbMATH
  • Middle East Technical University Affiliated: Yes


We study the order ten polynomial equation satisfied by the radius of the spherical timelike orbits for a massive particle with a generic energy around a Kerr black hole. Even though the radii of the prograde and retrograde orbits at the equatorial or polar plane for particles with zero or unit energy have a monotonic dependence on the rotation parameter of the black hole, we show that there is a critical inclination angle above which the retrograde orbits have a nonmonotonic dependency on the rotation of the black hole. Thus the capture cross section of these retrograde orbits decreases with increasing black hole spin. Hence their efficiency to reduce the black hole's spin is decreased. We also provide an equation for the critical inclination angle that shows exactly at which point the nonmonotonicity starts. In addition, we employ the Lagrange-Burmann method to find approximate analytic solutions from the known exact solutions.