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