Superradiance in the sky

TL;DR

This paper explores how electromagnetic and gravitational waves can be amplified by rotating black holes through superradiance, potentially leading to observable effects in astrophysical systems like binary and triple systems.

Contribution

It identifies specific conditions under which superradiant amplification occurs and discusses potential observable signatures in astrophysical systems.

Findings

Superradiance can amplify low-frequency waves near black holes.

Observable orbital modulations due to superradiance are predicted.

Potential for testing general relativity through astrophysical observations.

Abstract

We discuss the conditions under which plane electromagnetic and gravitational waves can be amplified by a rotating black hole due to superradiant scattering. We show, in particular, that amplification can occur for low-frequency waves with an incidence angle parametrically close to 0 (or $\pi$) with respect to the black hole spin axis and with a parametrically small left (or right) polarization. This is the case of the radiation emitted by a spinning electric/magnetic dipole or gravitational quadrupole orbiting a black hole companion at large radius and co-rotating with the latter. This can yield observable effects of superradiance, for example, in neutron star-stellar mass black hole binaries, as well as in triple systems composed by a compact binary orbiting a central supermassive black hole. Due to superradiance, the total source luminosity in these systems exhibits a characteristic orbital modulation that may lead to significant observational signatures, thus paving the way for testing, in the near future, one of the most peculiar predictions of general relativity.