Fast ejecta from neutron star mergers and ultraviolet kilonova precursors

Electromagnetic counterparts to neutron star mergers detected in gravitational waves enhance localization accuracy and provide complementary information not contained in the gravitational waves. The primary optical / infrared counterpart of a neutron star merger is the r-process powered kilonova, which arises from sub-relativistic ejecta and evolves on timescales of days to weeks. An ultraviolet precursor that peaks on timescales of hours after the merger has been predicted to occur if a fraction of the ejecta moves sufficiently fast for neutrons in it to avoid capture, thus freezing out the r-process. Such fast ejecta can in principle originate from the contact interface between the two stars, but has thus far only been seen in smoothed-particle-hydrodynamic simulations of neutron star coalescence. Here I report results of time-dependent hydrodynamic simulations on a grid-based code which examine the existence of this fast ejecta and its parameter dependencies, if present.