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

Abstract

ABSTRACT 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 $M_{\rm n} \gtrsim 10^{-4}\, {\rm M_{\odot }}$, 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.