Testing Rotating Regular Metrics as Candidates for Astrophysical Black Holes

TL;DR

This paper investigates whether rotating regular black holes, which are non-Kerr solutions, can mimic Kerr black hole shadows within observational uncertainties, thus serving as viable astrophysical black hole candidates.

Contribution

It systematically analyzes the shadow similarities of three regular black hole models with Kerr black holes, considering current observational constraints from the Event Horizon Telescope.

Findings

Shadows of certain regular black holes are indistinguishable from Kerr within current uncertainties.

Specific parameter ranges for regular black holes are consistent with M87* observations.

Regular black holes could be viable alternatives to Kerr black holes as astrophysical candidates.

Abstract

The Event Horizon Telescope, a global submillimeter wavelength very long baseline interferometry array, produced the first image of supermassive black hole M87* showing a ring of diameter $\theta_d= 42\pm 3\,\mu$as, inferred a black hole mass of $M=(6.5 \pm 0.7) \times 10^9 M_\odot $ and allowed us to investigate the nature of strong-field gravity. The observed image is consistent with the shadow of a Kerr black hole, which according to the Kerr hypothesis describes the background spacetimes of all astrophysical black holes. The hypothesis, a strong-field prediction of general relativity, may be violated in the modified theories of gravity that admit non-Kerr black holes. Here, we use the black hole shadow to investigate the constraints when rotating regular black holes (non-Kerr) can be considered as astrophysical black hole candidates, paying attention to three leading regular black hole models with additional parameters $g$ related to nonlinear electrodynamics charge. Our interesting results based on the systematic bias analysis are that rotating regular black holes shadows may or may not capture Kerr black hole shadows, depending on the values of the parameter $g$. Indeed, the shadows of Bardeen black holes ($g\lesssim 0.26 M$), Hayward black holes ($g\lesssim 0.65 M$), and non-singular black holes ($g\lesssim 0.25 M$) are indistinguishable from Kerr black hole shadows within the current observational uncertainties, and thereby they can be strong viable candidates for the astrophysical black holes. Whereas Bardeen black holes ( $g\leq 0.30182M$), Hayward black holes ($g\leq 0.73627M$), and non-singular black holes ($g\leq 0.30461M$), within the $1\sigma$ region for $\theta_d= 39\, \mu$as, are consistent with the observed angular diameter of M87*.