Constraining rotating black hole via curvature radius with observations of M87*

TL;DR

This paper uses the curvature radius derived from M87* shadow observations to constrain black hole parameters, effectively ruling out certain spins and charges, and demonstrating the method's potential in black hole studies.

Contribution

It introduces a novel approach using local curvature radius to constrain Kerr and Kerr-Newman black hole parameters based on shadow observations.

Findings

High spin and large inclination Kerr black holes are ruled out.

Distinct constrained patterns for low and high spins in Kerr-Newman black holes.

Low spin and charge black holes are consistent with constraints.

Abstract

Combined with the observation of M87*, shadow has gradually became a promising test of the black hole nature. Recently, EHT collaboration gave new constraints on the shadow size at 68\% confidence levels. In this work, we consider the new constrains on the black hole spin and charge for the Kerr and Kerr-Newmann black holes via the local curvature radius. For the Kerr black holes, the cases with high spin and large inclination angle are ruled out. For the Kerr-Newmann black holes, two new characteristic constrained patterns for low and high black hole spins are given. Near extremal black holes are always excluded unless for the high spin and low inclination angle. Moreover, we find that low black hole spin and charge can pass these constraints as expected. These results suggest that this approach of curvature radius is effective on constraining black hole parameters. We expect the local concept of the curvature radius will play a more important role on the study of the black hole shadow in the further.