Epicyclic oscillations of non-slender fluid tori around Kerr black holes

TL;DR

This paper derives relativistic formulas for epicyclic oscillations of pressure-supported fluid tori around Kerr black holes, revealing pressure effects lower oscillation frequencies and impacting high-frequency QPO models.

Contribution

It provides the first second-order accurate relativistic analysis of epicyclic modes in non-slender tori, including pressure effects and their influence on oscillation frequencies.

Findings

Pressure lowers epicyclic frequencies compared to test particles.

Results agree qualitatively with pseudo-Newtonian studies.

Implications for high-frequency QPO models are discussed.

Abstract

Considering epicyclic oscillations of pressure-supported perfect fluid tori orbiting Kerr black holes we examine non-geodesic (pressure) effects on the epicyclic modes properties. Using a perturbation method we derive fully general relativistic formulas for eigenfunctions and eigenfrequencies of the radial and vertical epicyclic modes of a slightly non-slender, constant specific angular momentum torus up to second-order accuracy with respect to the torus thickness. The behaviour of the axisymmetric and lowest-order ($m=\pm 1$) non-axisymmetric epicyclic modes is investigated. For an arbitrary black hole spin we find that, in comparison with the (axisymmetric) epicyclic frequencies of free test particles, non-slender tori receive negative pressure corrections and exhibit thus lower frequencies. Our findings are in qualitative agreement with the results of a recent pseudo-Newtonian study of analogous problem defined within the Paczy{\'n}ski-Wiita potential. Implications of our results on the high-frequency QPO models dealing with epicyclic oscillations are addressed.