Testing black hole superradiance with pulsar companions

TL;DR

This paper proposes that pulsar emissions can undergo superradiant scattering when near a spinning black hole, causing detectable luminosity modulations that could test general relativity.

Contribution

It introduces a novel mechanism for observing black hole superradiance through pulsar luminosity variations based on their orbital orientation.

Findings

Superradiant modes depend on pulsar's position relative to black hole.

Aligned spins enhance superradiant amplification at large angles.

Luminosity modulation can reach up to a few percent.

Abstract

We show that the magnetic dipole and gravitational radiation emitted by a pulsar can undergo superradiant scattering off a spinning black hole companion. We find that the relative amount of superradiant modes in the radiation depends on the pulsar's angular position relative to the black hole's equatorial plane. In particular, when the pulsar and black hole spins are aligned, superradiant modes are dominant at large angles, leading to an amplification of the pulsar's luminosity, whereas for small angles the radiation is dominantly composed of non-superradiant modes and the signal is attenuated. This results in a characteristic orbital modulation of the pulsar's luminosity, up to the percent level within our approximations, which may potentially yield a signature of superradiant scattering in astrophysical black holes and hence an important test of general relativity.