Abstract
Ultralight bosons can form large clouds around stellar-mass black holes via the superradiance instability. Through processes such as annihilation, these bosons can source continuous gravitational wave signals with frequencies within the range of LIGO and Virgo. If boson annihilation occurs, then the Galactic black hole population will give rise to many gravitational signals; we refer to this as the ensemble signal. We characterize the ensemble signal as observed by the gravitational-wave detectors; this is important because the ensemble signal carries the primary signature that a continuous wave signal has a boson annihilation origin. We explore how a broad set of black hole population parameters affects the resulting spin-0 boson annihilation signal and consider its detectability by recent searches for continuous gravitational waves. A population of $10^8$ black holes with masses up to $30\mathrm{M}_\odot$ and a flat dimensionless initial spin distribution between zero and unity produces up to a thousand signals loud enough to be in principle detected by these searches. For a more moderately spinning population the number of signals drops by about an order of magnitude, still yielding up to a hundred detectable signals for some boson masses. A non-detection of annihilation signals at frequencies between 100 and 1200 Hz disfavors the existence of scalar bosons with rest energies between $2\times10^{-13}$ and $2.5\times10^{-12}$ eV. Finally we show that, depending on the black hole population parameters, care must be taken in assuming that the continuous wave upper limits from searches for isolated signals are still valid for signals that are part of a dense ensemble: Between 200 and 300 Hz, we urge caution when interpreting a null result for bosons between 4 and $6\times10^{-13}$ eV.
Highlights
In the last few years, transient and rapidly evolving gravitational waves have been observed from the mergers of stellar-mass compact objects [1]
Other works have searched the gravitational-wave data for boson annihilation signals leveraging existing search pipelines and/or results from other astrophysical searches
All-sky O2 continuous wave survey nondetection results of [22] are used to exclude bosons with 1.1 × 10−13 eV < μb < 4 × 10−13 eV assuming that all black holes are formed with spin 0.998, or 1.2 × 10−13 eV < μb < 1.8 × 10−13 eV assuming that all black holes are formed with spin 0.6, and a range of 3M⊙–100M⊙ black hole masses in both cases
Summary
In the last few years, transient and rapidly evolving gravitational waves have been observed from the mergers of stellar-mass compact objects [1]. A well-motivated target is the axion, proposed to solve the strong-CP problem in particle physics [8,9,10]; axions and axionlike particles are promising dark matter candidates (see, e.g., [11] for a review) Bosons such as axions or axionlike particles can form “clouds” with enormous occupation numbers around rotating black holes, and they do so rapidly on astrophysical timescales when the black hole size is similar to the boson’s Compton wavelength [3,4]. In order to take all effects into account, we use simulated populations of 108 black holes and calculate the expected signal from all of the systems for bosons with energies between 1 × 10−13 and 4 × 10−12 eV, corresponding approximately to gravitational-wave frequencies between 50 and 2000 Hz. We investigate the detectability of the ensemble signal in current LIGO data by broad continuous gravitational-wave surveys [20,21,22,23], and its dependence on black hole population assumptions. We summarize the necessary background below and provide further details in the Appendix A; see, e.g., [4,12] for more details and [30] for a review
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