Axion clouds may survive the perturbative tidal interaction over the early inspiral phase of black hole binaries

Abstract

Gravitational wave observation has the potential of probing ultralight bosonic fields such as axion. Axion forms a cloud around a rotating black hole (BH) by superradiant instability and should affect the gravitational waveform from binary BHs. On the other hand, considering the cloud associated with a BH in a binary system, tidal interaction depletes the cloud in some cases during the inspiral phase. We made the exhaustive study of cloud depletion numerically in a wide parameter range for equal mass binaries, assuming only the quadrupolar tidal perturbation is at work. We found that clouds can avoid disappearing due to the tidal effect only when $l=1$ mode is the fastest growing mode and when the binary orbit is counter-rotating in the non-relativistic parameter region.