New method for black-hole spin measurement based on flux variation from an infalling gas ring

Understand

We propose a new method for black hole spin measurement.

• In this method, we consider a gas blob or ring falling onto a black hole from the marginally stable orbit, keeping its initial orbital angular momentum.
• We calculate the gas motion and photon trajectories in the Kerr space-time and, assuming that the gas blob or ring emits monochromatic radiation, carefully examine how it is observed by a distant observer.
• The light curve of the orbiting gas blob is composed of many peaks because of periodic enhancement of the flux due to the gravitational lensing and beaming effects.