Electromagnetic signals from the decay of free neutrons in the first hours of neutron star mergers

TL;DR

This paper models electromagnetic signals from free neutrons and electrons in neutron star mergers, predicting observable UV/optical emissions that can inform about ejecta properties and jet-cocoon dynamics.

Contribution

It introduces new semi-analytical models and 3D simulations for neutron decay and synchrotron emission in merger ejecta, expanding understanding of early electromagnetic signals.

Findings

High-latitude neutron decay enhances UV signals within 1 hour.

Synchrotron emission from decay electrons can produce hours-long UV/optical signals.

Observations can constrain ejected neutron mass and jet-cocoon properties.

Abstract

The first hours following a neutron star merger are considered to provide several UV/optical/NIR signals: $ \beta $-decay emission from free neutrons, radioactive decay of shocked heavy elements in the cocoon and cocoon's cooling emission. Here we consider two additional emission sources: $ \beta $-decay of free neutrons in the cocoon and synchrotron by the $ \beta $-decay electrons. We present 3D RHD simulations of jets that propagate in a multi-layer ejecta from the merger and calculate semi-analytically the resulting light curves. We find that the free neutrons emission at high latitudes is enhanced by the cocoon by a factor of a few to power a wide ($ \lesssim 60^\circ $) and brief ($ \sim 1 $ hour) UV signal that can reach an absolute magnitude of $\gtrsim$ -15, comparable with the cooling emission. If the ejected neutron matter mass is $ M_n \gtrsim 10^{-4}{\rm M_\odot} $, the synchrotron emission may yield a long ($ \sim 8 $ hours) quasi-isotropic UV/optical signal with an absolute magnitude between -12 and -15, depending on the magnetic field. Such a high mass of a mildly-relativistic component may partly obscure the cocoon's shocked r-process elements, thereby attenuating its radioactive decay emission. Future observations on these timescales, including null detections, may place constraints on the ejected neutron matter mass and shed light on the ejecta and jet-cocoon characteristics.