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 axions. Axions form 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 an exhaustive study of the cloud depletion numerically in a wide parameter range for equal mass binaries, assuming only the leading quadrupolar tidal perturbation is at work as a first step. Under this assumption, we found that clouds can avoid disappearing due to the tidal effect only when (1) the l = 1 mode, which refers to the lowest bound state with the azimuthal angular momentum eigenvalue being +1, is the fastest growing mode, (2) the binary orbit is counter-rotating relative to the BH spin and (3) the cloud is in the non-relativistic regime.