Free Neutron Decay in Kilonova Ejecta: X-ray/UV Flashes with Non-Thermal Effects

Neutron star-bearing compact-object mergers can create a kilonova, a transient powered by the radioactive decay of material synthesized via rapid neutron capture (r-process). During the merger, a fraction of the ejecta is launched at high speeds (0.4c) that can lead to neutrons evading capture onto seed nuclei, resulting in free neutrons. The free neutrons then decay and inject additional energy that power early-time (1 day) emission. Here, using Sedona, we present a grid of the first non-local thermodynamic equilibrium and frequency-dependent opacity radiative transfer simulations to predict early X-ray/UV/optical emission from free neutron decay that span the expected theoretical range of free neutron mass, mixing with r-process material, and extent of the high-velocity ejecta tail. The emission can be characterized by an SED that rapidly shifts from X-rays of about times10 41-10 42 erg s-1 in the first about minutes to a far-UV and near-UV peak on the timescale of about minutes to hours, followed by enhanced optical and IR emission for sufficiently large free neutron masses. The properties of the free neutron ejecta are most distinguishable, in principle, at extreme-UV and far-UV wavelengths, with SED peak flux and wavelength determined by the maximum velocity of the ejecta and the mixing. In the far-UV and near-UV, free neutron ejecta masses as small as 10-7 M exhibit a unique bump compared to neutron-free models, demonstrating the importance of upcoming UV missions like UVEX and ULTRASAT to constraining the nucleosynthetic environment of neutron star mergers.