Towards Robust Constraints on Axion Dark Matter using PSR J1745-2900

TL;DR

This paper uses advanced plasma ray-tracing techniques combined with observational data to set the most robust limits on axion-photon coupling in the galactic centre, reducing previous uncertainties related to emission direction.

Contribution

It introduces novel plasma ray-tracing methods to improve constraints on axion dark matter using neutron star magnetosphere observations.

Findings

Ray-tracing reduces sensitivity to emission direction uncertainties.

Established limits comparable to CAST experiment in the 4.2-60 μeV mass range.

Dark matter density uncertainties dominate the limits' robustness.

Abstract

We apply novel, recently developed plasma ray-tracing techniques to model the propagation of radio photons produced by axion dark matter in neutron star magnetospheres and combine this with both archival and new data for the galactic centre magnetar PSR J1745-2900. The emission direction to the observer and the magnetic orientation are not constrained for this object leading to parametric uncertainty. Our analysis reveals that ray-tracing greatly reduces the signal sensitivity to this uncertainty, contrary to previous calculations where there was no emission at all in some directions. Based on a Goldreich-Julian model for the magnetosphere and a Navarro-Frank-White model for axion density in the galactic centre, we obtain the most robust limits on the axion-photon coupling, to date. These are comparable to those from the CAST solar axion experiment in the mass range $\sim 4.2-60\,\mu{\rm eV}$. If the dark matter density is larger, as might predicted by a "spike" model, the limits could be much stronger. The dark matter density in the region of the galactic centre is now the biggest uncertainty in these calculations.