Neutron-powered precursors of kilonovae

TL;DR

This paper predicts a neutron decay-powered optical precursor to kilonovae, which occurs hours after neutron star mergers and can reveal details about the merger and neutron star properties.

Contribution

It introduces the concept of a neutron-powered precursor to kilonovae, highlighting its potential as an observable signature and its implications for understanding neutron star mergers.

Findings

Neutron decay in ejecta produces a bright, blue precursor observable hours after merger.

The precursor peaks at U-band magnitude ~22 at 200 Mpc distance.

The precursor's properties are robust against moderate leptonization effects.

Abstract

The merger of binary neutron stars (NSs) ejects a small quantity of neutron rich matter, the radioactive decay of which powers a day to week long thermal transient known as a kilonova. Most of the ejecta remains sufficiently dense during its expansion that all neutrons are captured into nuclei during the r-process. However, recent general relativistic merger simulations by Bauswein and collaborators show that a small fraction of the ejected mass (a few per cent, or ~1e-4 Msun) expands sufficiently rapidly for most neutrons to avoid capture. This matter originates from the shocked-heated interface between the merging NSs. Here we show that the beta-decay of these free neutrons in the outermost ejecta powers a `precursor' to the main kilonova emission, which peaks on a timescale of a few hours following merger at U-band magnitude ~22 (for an assumed distance of 200 Mpc). The high luminosity and blue colors of the neutron precursor render it a potentially important counterpart to the gravitational wave source, that may encode valuable information on the properties of the merging binary (e.g. NS-NS versus NS-black hole) and the NS equation of state. Future work is necessary to assess the robustness of the fast moving ejecta and the survival of free neutrons in the face of neutrino absorptions, although the precursor properties are robust to a moderate amount of leptonization. Our results provide additional motivation for short latency gravitational wave triggers and rapid follow-up searches with sensitive ground based telescopes.