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

Ore Gottlieb, Abraham Loeb

Research output: Contribution to journalArticlepeer-review


The first hours following a neutron star merger are considered to provide several ultraviolet (UV)/optical/near-infrared signals: β-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: β-decay of free neutrons in the cocoon and synchrotron by the β-decay electrons. We present three-dimensional relativistic hydrodynamic 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 (60°) and brief (∼1 h) UV signal that can reach an absolute magnitude of -15, comparable with the cooling emission. If the ejected neutron matter mass is, the synchrotron emission may yield a long (∼8 h) 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 time-scales, including null detections, may place constraints on the ejected neutron matter mass and shed light on the ejecta and jet-cocoon characteristics.

Original languageEnglish
Pages (from-to)1753-1760
Number of pages8
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - 1 Apr 2020


  • methods: numerical
  • radiation mechanisms:general


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