# Axion superradiance in rotating neutron stars

## TL;DR

This paper introduces a novel superradiant instability mechanism for axion bound states in neutron star magnetospheres, driven by axion-photon mixing and stellar resistivity, with potential observational implications for pulsar spin-down.

## Contribution

It presents the first analysis of axion superradiance in neutron stars, highlighting a new instability mechanism involving magnetospheric resistivity and mode mixing.

## Key findings

- Derived superradiant eigenfrequencies for axion-photon system.
- Compared instability timescales with pulsar spin-down data.
- Identified conditions for axion-induced energy extraction from neutron stars.

## Abstract

It is a well-known fact that compact gravitating objects admit bound state configurations for massive bosonic fields. In this work we describe a new class of superradiant instabilities of axion bound states in neutron star magnetospheres. The instability arises from the mixing of axion and photon modes in the magnetic field of the neutron star which extract energy from the rotating magnetosphere. Unlike for black holes, where the dissipation required for superradiance is provided by an absorbative horizon, the non-hermitian dynamics in this paper come from the resistivity in the stellar magnetosphere arising from a finite bulk conductivity. The axion field mixes with photon modes which superradiantly scatter off the magnetosphere, extracting rotational energy which is then deposited back into the axion sector leading to an instability. We derive the superradiant eigenfrequencies for the axion-photon system using quantum mechanical perturbation theory on the axion boundstate, drawing an analogy with atomic selection rules. We then compare the characteristic time scale of the instability to the spin-down measurements of pulsars which limit the allowed rate of angular momentum extraction from neutron stars.