Strong Gravity Effects of Rotating Black Holes: Quasiperiodic Oscillations

TL;DR

This paper investigates the strong gravity effects on geodesic motion around rotating black holes, analyzing fundamental frequencies and their relation to observed quasiperiodic oscillations, including in braneworld gravity scenarios.

Contribution

It provides a detailed numerical analysis of epicyclic frequencies in Kerr and braneworld black holes, linking theoretical predictions to observed QPOs and exploring effects beyond general relativity.

Findings

Epicyclic frequencies match observed QPOs in some black hole binaries.

Maxima of radial and vertical frequencies exhibit a 2:1 ratio near extreme spins.

Braneworld black holes with negative tidal charge are consistent with QPO observations.

Abstract

We explore strong gravity effects of the geodesic motion in the spacetime of rotating black holes in general relativity and braneworld gravity. We focus on the description of the motion in terms of three fundamental frequencies: The orbital frequency, the radial and vertical epicyclic frequencies. For a Kerr black hole, we perform a detailed numerical analysis of these frequencies at the innermost stable circular orbits and beyond them as well as at the characteristic stable orbits, at which the radial epicyclic frequency attains its highest value. We find that the values of the epicyclic frequencies for a class of stable orbits exhibit good qualitative agreement with the observed frequencies of the twin peaks quasiperiodic oscillations (QPOs) in some black hole binaries. We also find that at the characteristic stable circular orbits, where the radial (or the vertical) epicyclic frequency has maxima, the vertical and radial epicyclic frequencies exhibit an approximate 2 : 1 ratio even in the case of near-extreme rotation of the black hole. Next, we perform a similar analysis of the fundamental frequencies for a rotating braneworld black hole and argue that the existence of such a black hole with a negative tidal charge, whose angular momentum exceeds the Kerr bound in general relativity, does not confront with the observations of high frequency QPOs.