Transient Radio Signatures from Neutron Star Encounters with QCD Axion Miniclusters

The QCD axion is expected to form dense structures known as axion miniclusters if the Peccei-Quinn symmetry is broken after inflation. Miniclusters that have survived until today will interact with neutron stars (NSs) in the Milky Way to produce transient radio signals from axion-photon conversion in the NS magnetosphere. We quantify the properties of these encounters and find that they occur frequently [$\mathcal{O}(1–100){\mathrm{day}}^{-1}$], last between a day and a few months, are spatially clustered toward the Galactic Center, and can reach observable fluxes. These radio transients are within reach of current generation telescopes and therefore offer a promising pathway to discovering QCD axion dark matter.

Figure 1

AMC-NS encounter rate incorporating the effects of AMC disruptions due to stellar encounters. We show results assuming that AMCs are solely on circular orbits (dashed) and assuming a distribution of orbital eccentricities (dotted). The vertical dotted line marks the position of the Solar System, $r_{\odot }$.

Figure 2

Mean flux density and duration of the radio signal for a random sample of ${10}^4$ AMC-NS encounters, colored by mean AMC density. Overlaid are 1-, 2-, 3-, and $4\text{−}\sigma$ contours for the distribution of $⟨\mathcal{S}⟩$ and $T_{\mathrm{enc}}$, derived from a larger sample of ${10}^7$ simulations. The diagonal dashed lines show the minimum $5\sigma$ detectable flux estimated using the radiometer equation [92] (where we assume that the observation time equals the duration of the encounter) for the VLA [88], SKA1-mid [93, 94], and SKA2 [24]. We assume AMCs have power-law (left) or NFW (right) internal density profiles.

Figure 3

Expected distribution of all AMC-NS encounters (top) and bright AMC-NS encounters (bottom), assuming NFW internal AMC density profiles. For AMCs with PL density profiles, 68% of all encounters lie within 7° of the Galactic Center.