Axion superradiance

TL;DR

The paper addresses ultra-light bosons, notably axions, and their potential to trigger superradiant instabilities around rotating compact objects. It reviews black hole superradiance, deriving bound-state complex frequencies $\omega_{nlm}=\omega_R+i\omega_I$ and the condition $\omega_R < m \Omega$, with maximal growth near $GM\mu \sim 1$. It also develops a framework for stellar superradiance using damping rates $\Gamma_\phi$ in a worldline EFT and matching to classical results, enabling calculation of $\Gamma_{n\ell m}$ for generic bosons. The discussion highlights observational prospects via BH spin measurements and multi-messenger signals, the role of accretion and magnetospheres, and the broader significance of using astrophysical systems to probe ultra-light beyond-Standard-Model physics.

Abstract

Light bosonic fields may suffer an instability around a rotating compact object. This process, known as superradiance, leads to the exponential amplification of the field around a black hole or neutron star, while the spin of the central object is correspondingly depleted. The discovery of a highly spinning black hole could therefore be used to constrain the existence of light bosons such as axions in a particular range of masses. These constraints apply for very low non-gravitational couplings between the boson and the Standard Model, offering a powerful search strategy for new physics. However, care must be taken to include the more complex effects of the black hole's astrophysical environment. Conversely, stellar superradiance could allow us to probe additional non-gravitational interactions between a new boson at the stellar matter. In this article, I will discuss the current status and future directions of axion superradiance. This is a contribution to the proceedings of the 3rd General Meeting of the COST Action COSMIC WISPers.