Simulating the Galactic population of axion clouds around stellar-origin black holes: Gravitational wave signals in the 10-100 kHz band

Ultralight scalar fields can experience runaway "superradiant"amplification near spinning black holes, resulting in a macroscopic "axion cloud,"which slowly dissipates via continuous monochromatic gravitational waves. For a particular range of boson masses, O(10-11-10-10) eV, an axion cloud will radiate in the 10-100 kHz band of the levitated sensor detector (LSD). Using fiducial models of the mass, spin, and age distributions of stellar-origin black holes, we simulate the present-day Milky Way population of these hypothetical objects. As a first step toward assessing the LSD's sensitivity to the resultant ensemble of gravitational wave signals, we compute the corresponding signal-to-noise ratios which build up over a nominal integration time of 107 s, assuming the projected sensitivity of the 1 m LSD prototype currently under construction, as well as for future 10 m and 100 m concepts. For a 100 m cryogenic instrument, hundreds of resolvable signals could be expected if the boson mass μ is around 3×10-11 eV, and this number diminishes with increasing μ up to ≈5.5×10-11 eV. The much larger population of unresolved sources will produce a confusion foreground which could be detectable by a 10-m instrument if μ∈(3-4.5)×10-11 eV or by a 100-m instrument if μ∈(3-6)×10-11 eV.