Black hole superradiance with (dark) matter accretion

TL;DR

This paper explores how matter accretion influences black hole superradiance, revealing conditions where accretion enhances the superradiance cloud, and introduces the concept of over-superradiance, with applications to dark matter and baryonic disks.

Contribution

It extends superradiance studies by analyzing the impact of environmental accretion, providing analytical descriptions of black hole evolution along the superradiance threshold, and introduces over-superradiance as a new mechanism.

Findings

Accretion can increase the superradiance cloud beyond 10% of black hole mass.

Black hole evolution can be described analytically when drifting along the superradiance threshold.

Explicit examples include accretion from wave dark matter vortices and baryonic disks.

Abstract

Studies of black hole superradiance often focus on the growth of a cloud in isolation, accompanied by the spin-down of the black hole. In this paper, we consider the additional effect of the accretion of matter and angular momentum from the environment. We show that, in many cases, the black hole evolves by drifting along the superradiance threshold, in which case the evolution of its parameters can be described analytically or semi-analytically. We quantify the conditions under which accretion can serve as a mechanism to increase the cloud-to-black hole mass ratio, beyond the standard maximum of about 10%. This occurs by a process we call over-superradiance, whereby accretion effectively feeds the superradiance cloud, by way of the black hole. We give two explicit examples: accretion from a vortex expected in wave dark matter and accretion from a baryonic disk. In the former case, we estimate the accretion rate by using an analytical fit to the asymptotic behavior of the confluent Heun function. Level transition, whereby one cloud level grows while the other shrinks, can be understood in a similar way.