Axion-Photon Conversion Signals from Neutron Stars with Spacetime Curvature Accounted for in the Magnetosphere Model

Abstract

Axions are a well-motivated dark matter candidate. They may be detectable from radio line emission due to resonant conversion in neutron star magnetospheres. While radio data collection for this signal has begun, further efforts are required to solidify the theoretical predictions for the resulting radio lines. Usually, the flat spacetime Goldreich-Julian model of the neutron star magnetosphere is used, while a Schwarzschild geometry is assumed for the ray tracing. We assess the impact of incorporating the spacetime curvature into the magnetosphere model. We examine a range of neutron star and axion masses and find an average difference of $37\%$ and $22\%$ in radiated power compared to the standard Goldreich-Julian magnetosphere model for a $1\mu eV$ and $10\mu eV$ mass axion, respectively, in the case of a $2.2M_\odot$ mass neutron star. A much lesser difference is found for lower-mass neutron stars, as in that case axion-photon conversion occurs further from the Schwarzschild radius.