Axion cloud evaporation during inspiral of black hole binaries -- the
  effects of backreaction and radiation
  Ultralight scalar fields such as axions can form clouds around rotating black
holes (BHs) by the superradiant instability. It is important to consider the
evolution of clouds associated with BH binaries for the detectability of the
presence of clouds through gravitational wave signals and observations of the
mass and spin parameters of BHs. We re-examine the impact on the axion cloud
due to the tidal perturbation from the companion in a binary system taking into
account the following points. First, we study the influence of higher-multipole
moments. Second, we consider the backreaction due to the angular momentum
transfer between the cloud and the orbital motion. This angular momentum
transfer further causes the backreaction to the hyperfine split through the
change in geometry. Finally, we calculate the particle number flux to the
infinity induced by the tidal interaction. As a result, we found that the
scalar field is not reabsorbed by the BH. Instead, the scalar particles are
radiated away to evaporate during the inspiral, irrespectively of the direction
of the orbital motion, for almost equal mass binaries.
