Kerr black hole superradiance and gravitational lensing as a measure of ultralight bosons mass

Abstract

The effects of Kerr BH superradiance alongside ultralight boson fields has been poorly studied within electromagnetic radiation. Motivated by the Event Horizon Telescope results, we explore the observational implications using gravitational lensing. In order to do so, we review some key concepts like Detweiler's approximation and gravitational lensing. Later, we provide a fitted formula for the spin evolution and for measuring the spin of a Kerr black hole in terms of the shadow for an equatorial observer. We find that the spindown and mass change due to superradiance instability can be observable by changes in the black hole shadow, leaving a characteristic imprint and therefore making it distinguishable from other possible spindown processes. Furthermore, assuming one can measure the mass of the black hole, we develop a method of measuring the ultralight boson mass by means of measuring the angular diameter of the black hole shadow.