Observational Signature of High Spin at the Event Horizon Telescope

TL;DR

This paper analytically predicts the unique observational signatures of high-spin black holes near the event horizon, highlighting distinctive image motions and caustic structures that could confirm high spin in astrophysical observations.

Contribution

It provides an analytical model of the observational appearance of sources near high-spin black holes, revealing distinctive image behaviors and caustic features.

Findings

Primary image moves on a vertical line segment

Secondary images exhibit complex caustic structures

Unique signatures could confirm high black hole spin

Abstract

We analytically compute the observational appearance of an isotropically emitting point source on a circular, equatorial orbit near the horizon of a rapidly spinning black hole. The primary image moves on a vertical line segment, in contrast to the primarily horizontal motion of the spinless case. Secondary images, also on the vertical line, display a rich caustic structure. If detected, this unique signature could serve as a "smoking gun" for a high-spin black hole in nature.