Gravitational wave emission from collisions of compact scalar solitons

Abstract

We numerically investigate the gravitational waves generated by the head-on collision of equal-mass, self-gravitating, real scalar field solitons (oscillatons) as a function of their compactness C. We start with solitons that are initially at rest with respect to each other and show that there exist three different possible outcomes resulting from their collisions: (1) an excited stable oscillaton for low C, (2) a merger and formation of a black hole for intermediate C, and (3) a premerger collapse of both oscillatons into individual black holes for large C. For (1), the excited, aspherical oscillaton continues to emit gravitational waves. For (2), the total energy in gravitational waves emitted increases with compactness and possesses a maximum which is greater than that from the merger of a pair of equivalent mass black holes. The initial amplitudes of the quasinormal modes in the postmerger ringdown in this case are larger than that of collisions of corresponding mass black holes - potentially a key observable to distinguish black-hole mergers from their scalar mimics. For (3), the gravitational wave output is indistinguishable from a similar mass, black hole-black hole merger. Based on our results, LIGO may be sensitive to oscillaton collisions from light scalars of mass 10-12 eVm10-10 eV.