Jets and Rings in Images of Spinning Black Holes

TL;DR

This paper introduces a dual cone model for millimeter emission near spinning black holes, predicting observable spin-induced displacements of the photon ring and analyzing the impact of jet structure on image features.

Contribution

The paper presents a novel dual cone model for black hole emission, analyzing spin effects on images and proposing a method to detect black hole spin via jet and ring displacements.

Findings

Spin displaces the photon ring perpendicular to the spin axis projection.

Jet centerline can serve as a spin reference point.

Ring substructure varies with cone opening angle.

Abstract

We develop a "dual cone" model for millimeter wavelength emission near a spinning black hole. The model consists of optically thin, luminous cones of emission, centered on the spin axis, which are meant to represent jet walls. The resulting image is dominated by a thin ring. We first consider the effect of black hole's spin on the image, and show that the dominant effect is to displace the ring perpendicular to the projection of the spin axis on the sky by $2 a_* \sin i + \mathcal{O}(a_*^3)$. This effect is lower order in $a_*$ than changes in the shape and size of the photon ring itself, but is undetectable without a positional reference. We then show that the centerline of the jet can provide a suitable reference: its location is exactly independent of spin if the observer is outside the cone, and nearly independent of spin if the observer is inside the cone. If astrophysical uncertainties can be controlled for, then spin displacement is large enough to be detectable by future space VLBI missions. Finally, we consider ring substructure in the dual cone model and show that features in total intensity are not universal and depend on the cone opening angle.