Unique Multimessenger Signal of QCD Axion Dark Matter

We propose a multimessenger probe of QCD axion dark matter based on observations of black hole-neutron star binary inspirals. It is suggested that a dense dark matter spike may grow around intermediate mass black holes (${10}^3–{10}^5{\mathrm{M}}_{\odot }$). The presence of such a spike produces two unique effects: a distinct phase shift in the gravitational wave strain during the inspiral and an enhancement of the radio emission due to the resonant axion-photon conversion occurring in the neutron star magnetosphere throughout the inspiral and merger. Remarkably, the observation of the gravitational wave signal can be used to infer the dark matter density and, consequently, to predict the radio emission. We study the projected reach of the LISA interferometer and next-generation radio telescopes such as the Square Kilometre Array. Given a sufficiently nearby system, such observations will potentially allow for the detection of QCD axion dark matter in the mass range ${10}^{-7}\mathrm{eV}$ to ${10}^{-5}\mathrm{eV}$.

Figure 1

Illustration of the IMBH-DM-NS system. A DM halo of axions $a$ around the intermediate mass black hole (IMBH) produces a phase shift in the GW signal and radio emission due to its conversion into photons $\gamma$ in the neutron star (NS) magnetosphere.

Figure 2

Error on the DM density from GW measurements of $\mathit{\alpha }$. Green bands show $1\sigma$ uncertainties on the reconstructed DM density from analyzing the GW waveform (for a system at $d=0.01\mathrm{Gpc}$, representing a signal-to-noise ratio for LISA of $\sim 92$) over 10 bins in radius (measured from the position of the ${10}^4{\mathrm{M}}_{\odot }$ IMBH). The fiducial density profile is shown as a blue dashed line. Along the top axis, we also label the approximate time-to-merger in the vacuum case.

Figure 3

Projected reach in axion-photon coupling from radio observations. SKA2 sensitivity (100 hours) to the axion-photon coupling for different orbital separations $r$ and IMBH-DM-NS distances $d$, assuming $\theta =90°$. The QCD axion parameter space is represented by the blue band, while the vertical and horizontal gray bands show the ADMX [30, 31] and CAST [32] limits, respectively.