Observational Signature of High Spin at the Event Horizon Telescope

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.

36 pages, 6 figures. v2: added a figure and improved section 4.1, matches published version. Related videos available at https://youtu.be/ZRfTj5JKPkA and https://youtu.be/K0jEa0Q9NJA