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Boson stars have attracted much attention in recent decades as simple,self-consistent models of compact objects and also as self-gravitatingstructures formed in some dark-matter scenarios. Direct detection of thesehypothetical objects through electromagnetic signatures would be unlikelybecause their bosonic constituents are not expected to interact significantlywith ordinary matter and radiation. However, binary boson stars might form andcoalesce emitting a detectable gravitational wave signal which mightdistinguish them from ordinary compact object binaries containing black holesand neutron stars. We study the merger of two boson stars by numericallyevolving the fully relativistic Einstein-Klein-Gordon equations for a complexscalar field with a solitonic potential that generates very compact bosonstars. Owing to the steep mass-radius diagram, we can study the dynamics andgravitational radiation from unequal-mass binary boson stars with mass ratiosup to $q\approx23$ without the difficulties encountered when evolving binaryblack holes with large mass ratios. Similar to the previously-studiedequal-mass case, our numerical evolutions of the merger produce either anonspinning boson star or a spinning black hole, depending on the initialmasses and on the binary angular momentum. We do not find any evidence ofsynchronized scalar clouds forming around either the remnant spinning blackhole or around the remnant boson stars. Interestingly, in contrast to theequal-mass case, one of the mechanisms to dissipate angular momentum is nowasymmetric, and leads to large kick velocities (up to a few $10^4\,{\rm km/s}$)which could produce wandering remnant boson stars. We also compare thegravitational wave signals predicted from boson star binaries with those fromblack hole binaries, and comment on the detectability of the differences withground interferometers.

Gravitational waves and kicks from the merger of unequal mass, highly compact boson stars