Axion signals from neutron star populations

TL;DR

This paper explores the potential of neutron star populations, especially near the Galactic Centre, as probes for axion dark matter, addressing uncertainties in population estimates and comparing detection sensitivities.

Contribution

It models axion signals from well-understood neutron star populations outside the Galactic Centre to circumvent uncertainties and compares these with signals from the Galactic Centre magnetar.

Findings

Population modeling outside the Galactic Centre is feasible using pulsar observations.

Sensitivity of wide-angle searches with MeerKAT and SKA-low is currently less competitive than existing constraints.

Galactic Centre magnetar signals could provide sensitivities comparable to the neutron star population signals.

Abstract

Neutron stars provide a powerful probe of axion dark matter, especially in higher frequency ranges where there remain fewer laboratory constraints. Populations of neutron stars near the Galactic Centre have been proposed as a means to place strong constraints on axion dark matter. One downside of this approach is that there are very few direct observations of neutron stars in this region, introducing uncertainties in the total number of neutron stars in this ``invisible" population at the Galactic Centre, whose size must be inferred through birth rate modelling. We suggest this number could also be reduced due to stellar dynamics carrying stars away from the Galactic Centre via large kick velocities at birth. We attempt to circumvent the uncertainty on the Galactic Centre population size by modelling the axion signal from better understood populations outside the Galactic Centre using {\tt PsrPopPy} which is normalised against pulsar observations. We consider lower-frequency, wider-angle searches for this signal via a range of instruments including MeerKAT and SKA-low but find that the sensitivity is not competitive with existing constraints. Finally, returning to the Galactic Centre, we compare populations to single objects as targets for axion detection. Using the latest modelling of axion-photon conversion in the Galactic Centre magnetar, we conclude that within astrophysical uncertainties, the Galactic Centre population and the magnetar could give comparable sensitivities to axion dark matter, suggesting one should continue to search for both signals in future surveys.