Observational Signature of High Spin at the Event Horizon Telescope

Samuel E. Gralla, Alexandru Lupsasca, Andrew Strominger

Published 2017-10-30Version 1

We analytically compute the observational appearance of an isotropically emitting point source orbiting near 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.