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

TL;DR

This paper models the population of axion clouds around stellar black holes and assesses the gravitational wave signals in the 10-100 kHz band for current and future detectors, highlighting potential detectability of signals and confusion backgrounds.

Contribution

It provides the first simulation of Milky Way axion cloud populations and evaluates the sensitivity of LSD detectors to their gravitational wave signals.

Findings

Hundreds of resolvable signals could be detected with a 100-m detector at certain boson masses.

A confusion foreground from unresolved sources may be detectable with larger detectors.

Detectability depends on the axion mass and detector size, with specific ranges identified.

Abstract

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, $\mathcal{O}(10^{-11}$ -- $10^{-10})$ 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 towards assessing the LSD's sensitivity to the resultant ensemble of GW signals, we compute the corresponding signal-to-noise ratios which build up over a nominal integration time of $10^{7}$ 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 $\mu$ is around $3\times10^{-11}$ eV, and this number diminishes with increasing $\mu$ up to $\approx 5.5\times10^{-11}$ eV. The much larger population of unresolved sources will produce a confusion foreground which could be detectable by a $10$-m instrument if $\mu \in (3-4.5)\times10^{-11}$ eV, or by a $100$-m instrument if $\mu \in (3-6)\times10^{-11}$ eV.