Testing Ghasemi-Nodehi-Bambi spacetime with continuum-fitting method

TL;DR

This study investigates the continuum-fitting method's ability to test deviations from Kerr black holes using the Ghasemi-Nodehi-Bambi spacetime, revealing parameter degeneracies that hinder distinguishing non-Kerr metrics.

Contribution

It analyzes the impact of multiple parameters on accretion disk spectra and demonstrates the degeneracy in parameter constraints, highlighting challenges in testing alternative black hole metrics.

Findings

Most parameters are degenerate in spectral analysis.

Parameter b9 can be constrained at high spins.

Spectra of Kerr and Ghasemi-Nodehi-Bambi spacetimes are indistinguishable.

Abstract

The continuum-fitting method is the analysis of the thermal spectrum of the geometrically thin and optically thick accretion disk around stellar-mass black holes. A parametrization aiming to test the Kerr nature of astrophysical black holes was proposed in Ghasemi-Nodehi and Bambi in EPJC 76: 290, 2016. The metric contains 11 parameters in addition to the mass and spin parameters. One can recover the Kerr case by setting all parameters to one. In this paper, I study the continuum-fitting method in Ghasemi-Nodehi-Bambi background. I show the impact of each of the parameters on the spectra. I then employ \c{hi}2 studies and show that using the continuum-fitting method all parameters of Ghasemi-Nodehi- Bambi spacetime are degenerate. However, the parameter b9 can be constrained in the case of a high spin value and the Ghasemi-Nodehi-Bambi black hole as reference. This degeneracy means that the spectra of the Kerr case cannot be distinguished from spectra produced in Ghasemi-Nodehi-Bmabi spacetime. This is a problem as regards measuring the spin of astrophysical black holes and constrain possible deviations from the Kerr case of General Relativity.