Maximum observable blueshift from circular equatorial Kerr orbiters

The region of spacetime near the event horizon of a black hole can be viewed as a deep potential well at large gravitational redshift relative to distant observers. However, matter orbiting in this region travels at relativistic speeds and can impart a significant Doppler shift to its electromagnetic emission, sometimes resulting in a net observed blueshift at infinity. Thus, a black hole broadens the line emission from monochromatic sources in its vicinity into a smoothly decaying “red wing”—whose flux vanishes at large redshift—together with a “blue blade” that retains finite flux up to a sharp edge corresponding to the maximum observable blueshift. In the talk, I will describe the blue blade produced by isotropic monochromatic emitters on circular equatorial orbits around a Kerr black hole, and outline how the maximum blueshift simply encodes black hole spin and inclination. These results bear direct relevance to ongoing and future observations aiming to infer the angular momenta of supermassive black holes from the broadening of their surrounding line emission.