Inner Accretion Disk Edges in a Kerr-Like Spacetime

TL;DR

This paper investigates the properties of accretion disks in a Kerr-like spacetime, focusing on the inner edge determined by vertical instability, and explores how this affects observable iron line profiles.

Contribution

It analyzes the inner edge of accretion disks in a specific Kerr-like metric where the edge is set by vertical instability, providing new expressions and simulation results.

Findings

Inner disk edges can be set by vertical instability rather than ISCO.

Relativistically broadened iron lines vary significantly with spin and deviation parameters.

The model offers potential observational tests for deviations from Kerr spacetime.

Abstract

According to the no-hair theorem, astrophysical black holes are uniquely described by the Kerr metric. In order to test this theorem with observations in either the electromagnetic or gravitational-wave spectra, several Kerr-like spacetimes have been constructed which describe potential deviations from the Kerr spacetime in parametric form. For electromagnetic tests of the no-hair theorem, such metrics allow for the proper modeling of the accretion flows around candidate black holes and the radiation emitted from them. In many of these models, the location of the inner edge of the accretion disk is of special importance. This inner edge is often taken to coincide with the innermost stable circular orbit, which can serve as a direct probe of the spin and the deviation from the Kerr metric. In certain cases, however, an innermost stable circular orbit does not exist, and the inner edge of an accretion disk is instead determined by an instability against small perturbations in the direction vertical to the disk. In this paper, I analyze the properties of accretion disks in the Kerr-like metric proposed by Johannsen and Psaltis [Phys. Rev. D 83, 124015 (2011)], whose inner edges are located at the radii where this vertical instability occurs. I derive expressions of the energy and axial angular momentum of disk particles that move on circular equatorial orbits and calculate the locations of the inner disk edges. As a possible observable of such accretion disks, I simulate profiles of relativistically broadened iron lines and show that they depend significantly on the values of the spin and the deviation parameter.