Gravitational radiation from an axion cloud around a black hole: Superradiant phase

Understand

Motivated by possible existence of string axions with ultralight masses, we study gravitational radiation from an axion cloud around a rotating black hole (BH).

• The axion cloud extracts the rotation energy of the BH by superradiant instability, while it loses energy through the emission of gravitational waves (GWs).
• In this paper, GWs are treated as perturbations on a fixed background spacetime to derive the energy emission rate.
• We give an analytic approximate formula for the case where axion's Compton wavelength is much larger than the BH radius, and then, present numerical results without approximation.