Gravitational atoms: Gravitational radiation from excited boson stars

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Abstract
Boson stars are gravitationally bound states of complex scalar fields. In general, as a solution of the Einstein-Klein-Gordon system, the scalar field can be expanded as a sum over modes given by the product of radial functions and spherical harmonics. We show that if the configuration is not in its ground state, the excited modes will decay to the ground state through emission of scalar and gravitational radiation. Using a Newtonian approximation, we obtain the power radiated in gravitational waves and the frequency of the gravitons emitted in the decay, in the case where most of the star is already in its ground state. The process is similar to the spontaneous emission of a photon in atoms, hence the name "gravitational atoms."