Note on the Cardoso-Pani-Rico parametrization to test the Kerr black hole hypothesis

TL;DR

This paper examines a new parametrization of black hole spacetimes proposed by Cardoso, Pani, and Rico, revealing it can describe significant deviations from Kerr black holes in accretion disk spectra, unlike previous models.

Contribution

It analyzes the Cardoso-Pani-Rico metric in the context of accretion disk spectra, highlighting its ability to capture large deviations from Kerr black holes that previous parametrizations missed.

Findings

The new metric can describe large deviations in the spacetime geometry.

Previous models like Johannsen-Psaltis missed some non-Kerr spacetimes.

Fast-rotating black hole candidates may have significant non-Kerr features.

Abstract

The construction of a generic parametrization to describe the spacetime geometry around astrophysical black hole candidates is an important step to test the Kerr black hole hypothesis. In the last few years, the Johannsen-Psaltis metric has been the most common framework to study possible deviations from the Kerr solution with present and near future observations. Recently, Cardoso, Pani and Rico have proposed a more general parametrization. The aim of the present paper is to study this new metric in a specific context, namely the thermal spectrum of geometrically thin and optically thick accretion disks. The most relevant finding is that the spacetime geometry around objects that look like very fast-rotating Kerr black holes may still have large deviations from the Kerr solution. This was not the case with the Johannsen-Psaltis metric, which means the latter was missing an important class of non-Kerr spacetimes.