Ring Asymmetry and Spin in M87*

TL;DR

This study compares EHT observations of M87*'s asymmetric ring with GRMHD models to constrain the black hole's spin, supporting the presence of nonzero spin and informing jet-launching theories.

Contribution

It provides the first observational constraints on M87*'s spin using EHT data and GRMHD models, linking spin to jet physics and black hole growth.

Findings

Strongly magnetized models marginally disfavor low spin values.

EHT data supports nonzero black hole spin consistent with jet models.

Future observations could tighten spin constraints.

Abstract

Event Horizon Telescope (EHT) images of the supermassive black hole M87* depict an asymmetric ring of emission. General relativistic magnetohydrodynamic (GRMHD) models of M87* and its accretion disk predict that the amplitude and location of the ring's peak brightness asymmetry should fluctuate due to turbulence in the source plasma. We compare the observed distribution of brightness asymmetry amplitudes to the simulated distribution in GRMHD models, across varying black hole spin $a_{*}$. We show that, for strongly magnetized (MAD) models, three epochs of EHT data marginally disfavor $|a_{*}| \lesssim 0.2$. This is consistent with the Blandford-Znajek model for M87's jet, which predicts that M87* should have nonzero spin. We show quantitatively how future observations could improve spin constraints, and discuss how improved spin constraints could distinguish between differing jet-launching mechanisms and black hole growth scenarios.