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Axion superradiance in rotating neutron stars

Accepted version
Peer-reviewed

Type

Article

Change log

Authors

Day, Francesca V 
McDonald, Jamie I 

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.

Description

Keywords

axions, dark matter theory, pulsar magnetosphere

Journal Title

JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS

Conference Name

Journal ISSN

1475-7516
1475-7516

Volume Title

Publisher

IOP Publishing

Rights

All rights reserved