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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.