Axion superradiance in rotating neutron stars

In this talk I describe a new class of superradiant instabilities in which ultralight scalar fields extract rotational energy from neutron stars. The instability arises from the mixing of scalar 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 setup come from the resistivity in the stellar magnetosphere arising from a finite bulk conductivity. This leads to a set of complex eigenfrequencies associated to the instability, which we derive using techniques analogous to quantum mechanical perturbation theory by treating scalar field bound states like a perturbed atomic state. We then compare the instability rate to the characteristic astrophysical time scales of neutron stars given by their spin-down times.