K$\alpha$ iron line profile from accretion disks around regular and singular exotic compact objects

TL;DR

This paper investigates the iron Kα line profiles from accretion disks around exotic compact objects with non-vacuum interiors, proposing that their spectral features differ from those of Kerr black holes, thus enabling observational distinction.

Contribution

It introduces a model for accretion disks around regular and singular exotic objects, analyzing their iron line profiles to differentiate them from Kerr black holes.

Findings

Exotic objects produce distinct iron line features without low-energy tails.

Super-massive black hole candidates with high spin are unlikely to be these exotic objects.

Iron line profiles can serve as observational signatures to distinguish compact object types.

Abstract

The nature of the super-massive black hole candidates in galactic nuclei can be tested by analyzing the profile of the K$\alpha$ iron line observed in their X-ray spectrum. In this paper, we consider the possibility that the spacetime in the immediate vicinity of these objects may be described by some non vacuum exact solutions of Einstein's equations resulting as the end-state of gravitational collapse. The vacuum far away portion of the spacetime is described by the Schwarzschild metric, while the interior part may be either regular or have a naked singularity at the center. The iron line generated around this class of objects has specific features, which can be used to distinguish such objects from Kerr black holes. In particular, their iron line cannot have the characteristic low-energy tail of the line generated from accretion disks around fast-rotating Kerr black holes. We can thus conclude that the super-massive black hole candidates whose spin parameter has been estimated to be close to 1 assuming the Kerr background cannot be this kind of objects.