Abstract
Motivated by the 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 the axion Compton wavelength is much larger than the BH radius, and then present numerical results without approximation. The energy loss rate of the axion cloud through GW emission turns out to be smaller than the energy gain rate of the axion cloud by superradiant instability until nonlinear self-interactions of axions become important. In particular, an axion bosenova must happen at the last stage of superradiant instability.
Highlights
In a few years, the ground-based detectors, Advanced LIGO, Advanced VIRGO, and KAGRA, are expected to begin operation with sufficiently high sensitivity to detect gravitational wave (GW) signals from binary mergers of black holes (BHs) or neutron stars
We have studied GW emissions from an axion cloud in the superradiant phase around a rotating BH by combining analytic methods and numerical calculations
This formula cannot be used when αg becomes of the order of unity, because the axion cloud approaches the BH horizon and relativistic effects become significant
Summary
The ground-based detectors, Advanced LIGO, Advanced VIRGO, and KAGRA, are expected to begin operation with sufficiently high sensitivity to detect gravitational wave (GW) signals from binary mergers of black holes (BHs) or neutron stars. This can happen even for an axion cloud occupying just one bound-state level, and GWs with the frequency 2ω are emitted, where ω is the (real part of) the axion bound-state frequency For both processes, they derived approximate formulas for the GW radiation rates and concluded that the radiation rates are sufficiently small to allow the growth of the axion cloud by the superradiant instability until the occurrence of a bosenova. We perform numerical calculations of the GW emission in the Kerr background for general values of M μ This is certainly necessary because the superradiant instability for the modes l = m = 2 and 3 becomes effective when M μ ∼ 1, where the approximation M μ ≪ 1 breaks down.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
