Towards robust constraints on axion dark matter using PSR J1745-2900

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 center 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 (GJ) model for the magnetosphere and a Navarro-Frenk-White model for axion density in the Galactic center, 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 4. 2--60. If the dark matter density is larger, as might predicted by a ``spike'' model, the limits could be much stronger. For a fixed GJ magnetosphere model, we conclude that of the two principle uncertainties considered in this work---observing angle and dark matter density---the latter is the larger of the two in our present calculations.