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

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 $M_n\gtrsim 10^{-4}{\mathrm{M}}_{\odot }$, 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.

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Cited by in corpus (11)

• First Multimessenger Observations of a Neutron Star Merger
• Simulations of early kilonova emission from neutron star mergers
• Shocked jets in CCSNe can power the zoo of fast blue optical transients
• On the jet-ejecta interaction in 3D GRMHD simulations of binary neutron star merger aftermath
• The role of jet-cocoon mixing, magnetization and shock breakout in neutrino and cosmic-ray emission from short GRBs
• Cocoon breakout and escape from the ejecta of neutron star mergers
• Electromagnetic Follow-up Observations of Binary Neutron Star Mergers with Early Warnings from Decihertz Gravitational-wave Observatories
• r-Process Nucleosynthesis and Radioactively Powered Transients from Magnetar Giant Flares
• Prospects of Gravitational Wave Follow-up Through a Wide-field Ultra-violet Satellite: a Dorado Case Study
• Electromagnetic Follow-up to Gravitational Wave Events with the UltraViolet EXplorer (UVEX)
• Resolving the fastest ejecta from binary Neutron Star mergers: implications for electromagnetic counterparts