Degeneracy in Accretion Disk Spectra from Naked Singularities and Kerr Black Holes: Application to the AGN MCG-06-30-15

TL;DR

This study investigates whether accretion disk spectra can distinguish naked singularities from black holes, finding a degeneracy that complicates spin measurements but suggests potential for identification with independent methods.

Contribution

It introduces a model for accretion disks around naked singularities and compares spectral fits with black hole models, revealing degeneracy in spectral signatures.

Findings

Kerr black holes and JMN-1 naked singularities produce similar spectra.

Spectral fits favor Kerr black holes over Schwarzschild black holes.

Degeneracy can be broken with independent spin measurements.

Abstract

Theoretical studies suggest that gravitational collapse can form either a black hole or a visible (naked) singularity. Identifying observational signatures that distinguish these two types of collapsed objects is a holy grail of physics. Here, we examine whether relativistic accretion disk spectra can provide such a test. We construct an additive table model for a thin accretion disk in the Joshi-Malafarina-Narayan (JMN-1) naked singularity geometry matched to a Schwarzschild exterior and fit it to NuSTAR X-ray data from the AGN MCG-06-30-15. Our results are compared with standard Kerr and Schwarzschild black hole models. We also include the relativistic reflection spectral component relxill. Despite their different underlying geometries, the spinning (Kerr) black hole and the non-spinning JMN-1 naked singularity provide similar spectral fits, which are significantly better fits than the Schwarzschild black hole. This degeneracy between the naked singularity and the Kerr black hole could lead to incorrect spin measurements of collapsed objects using disk spectra. The degeneracy could be broken with an independent spin measurement, which could also help identify a naked singularity. Our results could also have a role in different spin distributions of collapsed objects measured from gravitational-wave sources and X-ray binaries.