Orientation of the crescent image of M 87*

TL;DR

This study uses simplified geometric and kinematic models of black hole accretion flows to interpret the crescent-shaped image of M 87* and explores the alignment between the black hole spin and the jet, highlighting the need for further multi-epoch observations.

Contribution

It introduces toy models for the emission regions around M 87* and compares simulated images with EHT observations to investigate the black hole's spin orientation and emission features.

Findings

Crescent emission can be explained by equatorial rings with specific flow patterns.

The observed brightness of the ESE sector remains unexplained by stationary models.

Multi-epoch and polarimetric data are crucial for confirming the black hole's spin alignment.

Abstract

The first image of the black hole (BH) M 87* obtained by the Event Horizon Telescope (EHT) has the shape of a crescent extending from the E to WSW position angles, while the observed direction of the large-scale jet is WNW. Images based on numerical simulations of BH accretion flows suggest that on average the projected BH spin axis should be oriented SSW. We explore highly simplified toy models for geometric distribution and kinematics of emitting regions in the Kerr metric, perform ray tracing to calculate the corresponding images, and simulate their observation by the EHT to calculate the corresponding visibilities and closure phases. We strictly assume that (1) the BH spin vector is fixed to the jet axis, (2) the emitting regions are stationary and symmetric with respect to the BH spin, and that (3) the emissivities are isotropic in the local rest frames. Emission from the crescent sector between SSE and WSW can be readily explained in terms of an equatorial ring with either circular or plunging geodesic flows, regardless of the value of BH spin. In the case of plane-symmetric polar caps with plunging geodesic flows, the dominant image of the cap located behind the BH is sensitive to the angular momentum of the emitter. Within the constraints of our model, we have not found a viable explanation for the observed brightness of the ESE sector. Most likely, the ESE "hotspot" has been produced by a non-stationary localised perturbation in the inner accretion flow. Alternatively, it could result from locally anisotropic synchrotron emissivities. Multi-epoch and polarimetric results from the EHT will be essential to verify the theoretically expected alignment of the BH spin with the large-scale jet.