Free Neutron Decay in Kilonova Ejecta: X-ray/UV Flashes with Non-Thermal Effects
Abstract
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 \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 ∼few ×1041−1042 erg s−1 in the first ∼ minutes to a far-UV and near-UV peak on the timescale of ∼ 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.
Related
- Spectra and Ionization Efficiencies of Charged Decay Particles in Kilonova Ejecta
- Beta-Particle Transport and Thermalization in Kilonova Ejecta with Detailed Atomic Microphysics
- Can we constrain the binary neutron star merger time from kilonova light curves?
- Spinning down neutron-star merger remnants with the Tayler-Spruit dynamo: Global simulations reveal the formation of massive disks and neutron-rich ejecta
- Spinning Down Neutron-star Merger Remnants with the Tayler–Spruit Dynamo: Global Simulations Reveal the Formation of Massive Disks and Neutron-rich Ejecta
BibTeX
@misc{brethauer2026free,
title = {{Free Neutron Decay in Kilonova Ejecta: X-ray/UV Flashes with Non-Thermal Effects}},
author = {Daniel Brethauer and Daniel Kasen and Smaranika Banerjee and Raffaella Margutti and Ryan Chornock},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.09311},
eprint = {2609.09311},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.09311},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Free Neutron Decay in Kilonova Ejecta: X-ray/UV Flashes with Non-Thermal Effects
AU - Daniel Brethauer
AU - Daniel Kasen
AU - Smaranika Banerjee
AU - Raffaella Margutti
AU - Ryan Chornock
PY - 2026
DA - 2026/09/08
PB - arXiv
DO - 10.48550/arxiv.2609.09311
UR - https://arxiv.org/abs/2609.09311
AB - 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 \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 ∼few ×10^41−10^42 erg s^−1 in the first ∼ minutes to a far-UV and near-UV peak on the timescale of ∼ 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.
ER -