Characterizing the continuous gravitational-wave signal from boson clouds around Galactic isolated black holes

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 gravitationalwave 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 30M ⊙ and a flat dimensionless initial spin distribution between zero and unity produces up to 1000 signals loud enough in principle to be 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 100 detectable signals for some boson masses. A nondetection of annihilation signals at frequencies between 100 and 1200 Hz disfavors the existence of scalar bosons with rest energies between 2 × 10 −13 and 2.5 × 10 −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 × 10 −13 and 6 × 10 −13 eV.

First search for a stochastic gravitational-wave background from ultralight bosons

“…as recently described in Ref. [71]), as our method to search for a SGWB (described in Sec. IV) is optimized for a Gaussian distributed signal.…”

“…We should also note that, for a given boson mass, the GW signals emitted by the galactic population would tend to accumulate in a very narrow frequency window around ω R [see Eq. (3)][71], unlike the extra-galactic component which should be spread over a broader range of frequencies due to the cosmological redshift. Our search method is better suited for signals that emit in a broad range of frequencies.…”

Application of a hierarchical MCMC follow-up to Advanced LIGO continuous gravitational-wave candidates

“…These yet-to-be detected signals are orders of magnitude weaker than compact binary coalescenses [1], requiring long integration times (months to years) to differentiate them from noise. Potentially detectable sources using the current generation of ground-based interferometric detectors, Advanced LIGO [2] and Advanced Virgo [3], are neutron stars (NSs) presenting some nonaxisymmetry such as crustal deformations, r-mode instabilities or free precession [4], or the annihilation of ultralight boson clouds around spinning black holes [5].…”

Search Methods for Continuous Gravitational-Wave Signals from Unknown Sources in the Advanced-Detector Era

“…The search for continuous gravitational-wave signals (CWs), long-duration forms of gravitational radiation, is one of the endeavours of gravitational-wave astronomy. These signals are produced by long-standing quadrupolar variations, such as rapidly spinning neutron star (NSs) sustaining a crustal deformation, undergoing an r-mode instability or in free precession [1][2][3][4], as well as more exotic sources such as the annihilation of ultra-light boson clouds around spinning black holes [5][6][7][8] or compact dark matter objects (CDOs) in the Solar System [9].…”