One-arm spiral instability in hypermassive neutron stars formed by dynamical-capture binary neutron star mergers

Using general-relativistic hydrodynamical simulations, we show that merging binary neutron stars can form hypermassive neutrons stars that undergo the one-arm spiral instability. We study the particular case of a dynamical capture merger where the stars have a small spin, as may arise in globular clusters, and focus on an equal-mass scenario where the spins are aligned with the orbital angular momentum. We find that this instability develops when postmerger fluid vortices lead to the generation of a toroidal remnant---a configuration whose maximum density occurs in a ring around the center-of-mass---with high vorticity along its rotation axis. The instability quickly saturates on a time scale of 10 ms, with the m=1 azimuthal density multipole mode dominating over higher modes. The instability also leaves a characteristic imprint on the postmerger gravitational wave signal that could be detectable if the instability persists in long-lived remnants.