Mergers of neutron stars (NS + NS) or neutron stars and stellar-mass black holes (NS + BH) eject a small fraction of matter with a subrelativistic velocity. Upon rapid decompression, nuclear-density medium condenses into neutron-rich nuclei, most of them radioactive. Radioactivity provides a long-term heat source for the expanding envelope. A brief transient has a peak luminosity in the supernova range, and the bulk of radiation in the UV-optical domain. In a paper published in Astrophysical Journal in 1998, Paczyński and I presented a very crude model of the phenomenon, and simple analytical formulae that can be used to estimate the parameters of a transient as a function of poorly known input parameters. The mergers may be detected with high-redshift supernova searches as rapid transients, many of them far away from the parent galaxies. It is possible that the mysterious optical transients detected by Schmidt et al. (1998) are related to neutron star mergers, since they typically have no visible host galaxy.
The above figure shows time variation of the bolometric luminosity of an expanding sphere generated by a neutron star merger, shown for models with a large mix of radioactive nuclides, which provide a heating rate inversely proportional to time from the beginning of expansion. The models have three values of the fraction of rest mass released as heat: f = 10-3, 10-4, 10-5, the mass M = 10-2M⊙, and the surface expansion velocity V = 1010 cm s-1. For the adopted opacity κ = 0.2 cm2 g-1, we have tc = 1.13 days (the critical time at which the expanding sphere becomes optically thin), as indicated with filled circles, separating the solid lines (the optically thick case) and the dashed lines (the optically thin case). [For details see L.-X. Li and B. Paczyński, Astrophys. J. 507, L59 (1998).]
The optical transient associated with a neutron star merger predicted by Paczyński and I in 1998 was finally observationally discovered in August 2017 during following-up observations of the gravitational wave and gamma-ray bust event GW170817/GRB170817A. The observed brightness and spectrum features agree with theoretical predictions remarkably well. [for a brief review see L.-X. Li, Astrophys. J. 872, 19 (2019)].