Origin of the ring ellipticity in the black hole images of M87*

TL;DR

This study analyzes the elliptical ring structure in M87* black hole images, attributing the observed ellipticity mainly to astrophysical effects like accretion flow turbulence rather than gravitational influences, using advanced imaging and simulations.

Contribution

It provides the first detailed measurement of ring ellipticity in M87* and compares it with extensive GRMHD simulations to identify dominant factors influencing the observed shape.

Findings

Ellipticity measured at approximately 0.08 with a position angle around 50 degrees.

No significant correlation between black hole spin and ellipticity.

Ellipticity correlates with non-ring emission, especially in non-thermal and jet-dominated models.

Abstract

We investigate the origin of the elliptical ring structure observed in the images of the supermassive black hole M87*, aiming to disentangle contributions from gravitational, astrophysical, and imaging effects. Leveraging the enhanced capabilities of the Event Horizon Telescope (EHT) 2018 array, including improved $(u,v)$-coverage from the Greenland Telescope, we measure the ring's ellipticity using five independent imaging methods, obtaining a consistent average value of $\tau = 0.08_{-0.02}^{+0.03}$ with a position angle $\xi = 50.1_{-7.6}^{+6.2}$ degrees. To interpret this measurement, we compare against General Relativistic Magnetohydrodynamic (GRMHD) simulations spanning a wide range of physical parameters including thermal or non-thermal electron distribution function, spins, and ion-to-electron temperature ratios in both low and high-density regions. We find no statistically significant correlation between spin and ellipticity in GRMHD images. Instead, we identify a correlation between ellipticity and the fraction of non-ring emission, particularly in non-thermal models and models with higher jet emission. These results indicate that the ellipticity measured from the \m87 emission structure is consistent with that expected from simulations of turbulent accretion flows around black holes, where it is dominated by astrophysical effects rather than gravitational ones. Future high-resolution imaging, including space very long baseline interferometry and long-term monitoring, will be essential to isolate gravitational signatures from astrophysical effects.