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

RESUMO

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 ($\gtrsim$ 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 ($\lesssim$1 day) emission. Here, using \texttt{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 $\sim\rm{few} \times10^{41}-10^{42}$ erg s$^{-1}$ in the first $\sim$ minutes to a far-UV and near-UV peak on the timescale of $\sim$ 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$_\odot$ 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.