Gravitational Wave Emission from Collisions of Compact Scalar Solitons
  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 $\mathcal{C}$. We show that
there exist three different possible outcomes for such collisions: (1) an
excited stable oscillaton for low $\mathcal{C}$, (2) a merger and formation of
a black-hole for intermediate $\mathcal{C}$, and (3) a pre-merger collapse of
both oscillatons into individual black-holes for large $\mathcal{C}$. 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
quasi-normal modes in the post-merger ring-down in this case are larger than
that of corresponding mass black-holes -- potentially a key observable to
distinguish black-hole mergers with their scalar mimics. For (3), the
gravitational wave output is indistinguishable from a similar mass,
black-hole--black-hole merger.
