Axion signals from neutron star populations

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.

💡 Research Summary

This paper investigates the prospects of detecting axion dark matter through its resonant conversion into photons in the magnetospheres of neutron stars (NSs). The authors focus on two complementary strategies: (1) using the population of “normal” pulsars distributed throughout the Milky Way, modelled with the open‑source tool PsrPopPy, and (2) targeting the Galactic Centre (GC) magnetar SGR J1745‑2900 as a single bright source.

The motivation stems from the fact that axion‑photon conversion is most efficient where the plasma frequency ω_P matches the axion mass m_a, a condition naturally realized in NS magnetospheres. The conversion probability depends on the surface magnetic field B₀, spin period P, magnetic inclination α, and the observer’s line‑of‑sight angle θ. For a single NS the emitted radio power L can be expressed analytically (following the Goldreich‑Julian model) and includes factors such as the local axion density enhancement due to gravitational focusing. However, a full ray‑tracing calculation for each orientation would be computationally prohibitive for a large population. The authors therefore average over all sky directions, showing that the total flux F received at Earth can be written as a sum over stars of L/|x|² weighted by the local NS density n_NS(x). The statistical error from orientation averaging scales as σ/√N, where N is the number of stars in the sample.

A major source of uncertainty is the “invisible” NS population that may reside within the central parsec of the Galaxy. Direct pulsar surveys have found essentially no normal pulsars in this region, so the total number N_GC must be inferred from birth‑rate models and assumptions about natal kick velocities. The authors argue that typical kick speeds of several hundred km s⁻¹ can eject a large fraction of newly born NSs from the deep GC potential well, leaving only a few percent of the originally formed stars in situ. This reduces the expected axion signal from the GC population relative to earlier optimistic estimates.

To bypass this uncertainty, the authors construct a well‑calibrated Galactic pulsar population using PsrPopPy, which reproduces the observed distributions of B₀, P, and α from the Parkes Multi‑Beam Survey (PMBS). They embed these synthetic pulsars in a realistic Galactic potential, assign each a dark‑matter density based on standard halo models, and compute the ensemble‑averaged axion‑induced radio flux. The resulting all‑sky signal is mapped in both L‑band (1–2 GHz) and C‑band (4–8 GHz).

The paper then evaluates the detectability of this diffuse signal with existing and planned radio facilities, including MeerKAT, SKA‑low, and bespoke low‑frequency arrays. Even under optimistic assumptions (full sky coverage, long integration times, and the most favorable axion masses), the constraints on the axion‑photon coupling g_{aγγ} derived from the Galactic pulsar population are weaker than current laboratory bounds (e.g., ADMX, CAST).

In contrast, the GC magnetar provides a point‑like, potentially much brighter source. Using the latest axion‑photon conversion calculations that incorporate magnetar‑specific plasma and magnetic‑field geometry, the authors find that the magnetar’s expected flux can be comparable to the integrated flux from the entire Galactic pulsar population, despite the latter’s larger number of sources. This equivalence holds within the large astrophysical uncertainties on both the magnetar’s magnetic field structure and the size of the hidden GC NS population.

The authors conclude that while the Galactic pulsar population alone is unlikely to yield competitive axion limits with current instruments, a combined search strategy that includes both the GC magnetar and the putative invisible GC NS population remains promising. Future work should aim to (i) refine models of star formation and natal kicks in the central parsec, (ii) improve magnetar magnetosphere modeling, and (iii) develop dedicated low‑frequency, wide‑field radio surveys capable of reaching the required sensitivities. Such efforts could close the gap between astrophysical and laboratory searches for axion dark matter in the µeV–tens‑of‑µeV mass range.