On the observational distinguishability of the Kerr and Kerr-Hayward metrics to EHT

TL;DR

This paper investigates whether the Kerr-Hayward metric, a singularity-free alternative to Kerr, can produce black hole images indistinguishable from Kerr in EHT observations, through simulations and extended analysis.

Contribution

It introduces a method to incorporate the Kerr-Hayward metric into EHT analysis, demonstrating that the two metrics can produce nearly identical observable signatures.

Findings

Kerr-Hayward metric can eliminate the curvature singularity.

Simulated images in Kerr-Hayward are functionally indistinguishable from Kerr.

EHT measurements may not distinguish between Kerr and Kerr-Hayward metrics.

Abstract

Astrophysical black holes appear well-represented by the Kerr metric, but this metric has the philosophical problem of a ring-like curvature singularity. We show that a phenomenological correction to the Kerr metric known as the Kerr-Hayward metric can eliminate the curvature singularity while preserving in detail many features of polarized black hole images now testable by the Event Horizon Telescope (EHT). To establish this, we produce new general relativistic magnetohydrodynamics (GRMHD) simulations of a magnetized plasma in a Kerr-Hayward spacetime, then we extend the EHT analysis framework to perform polarized radiative transfer in this spacetime. We detail our methodology for implementing this modified spacetime into an open-source pipeline. From fluid quantities such as the magnetic flux parameter and jet efficiency, to image quantities such as the polarization pattern and the photon ring structure, our results for the Kerr-Hayward metric appear functionally indistinguishable from the Kerr metric. Our study finds that under certain conditions, the singularity-free correction to the Kerr metric can yield observables that are effectively indistinguishable in EHT measurements.