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Abstract

Ultralight bosons can form large clouds around stellar-mass black holes via
the superradiance instability and produce continuous gravitational wave signals
with frequencies within the range of LIGO and Virgo. Unlike continuous
gravitational waves from neutron stars, boson annihilation signals are
clustered in frequency and have very small positive intrinsic frequency
derivatives. We characterize the expected spin-0 boson annihilation ensemble
signal from synthetic populations of isolated Galactic black holes. We explore
how the different population parameters affect the resulting 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. However, due to the high signal density between 200 and
300 Hz, we urge caution when interpreting a null result for bosons between $4$
and $6\times 10^{-13}$ eV. Finally, we emphasize that the ensemble signal
morphology would be the primary, perhaps sole, indicator that a continuous wave
signal has a boson annihilation origin.