The long-term evolution of neutron star merger remnants – II. Radioactively powered transients

Abstract

We use 3D hydrodynamic simulations of the long-term evolution of neutron star\nmerger ejecta to predict the light curves of electromagnetic transients that\nare powered by the decay of freshly produced r-process nuclei. For the dynamic\nejecta that are launched by tidal and hydrodynamic interaction, we adopt grey\nopacities of 10 cm$^2$/g, as suggested by recent studies. For our reference\ncase of a 1.3-1.4 $M_\\odot$ merger, we find a broad IR peak 2-4 d after the\nmerger. The peak luminosity is $\\approx 2\\times 10^{40}$ erg/s for an average\norientation, but increased by up to a factor of 4 for more favourable binary\nparameters and viewing angles. These signals are rather weak and hardly\ndetectable within the large error box (~100 deg$^2$) of a gravitational wave\ntrigger. A second electromagnetic transient results from neutrino-driven winds.\nThese winds produce `weak' r-process material with $50 < A < 130$ and abundance\npatterns that vary substantially between different merger cases. For an adopted\nopacity of 1 cm$^2$/g, the resulting transients peak in the UV/optical about 6\nh after the merger with a luminosity of $\\approx 10^{41}$ erg/s (for a wind of\n0.01 $M_\\odot$) These signals are marginally detectable in deep follow-up\nsearches (e.g. using Hypersuprime camera on Subaru). A subsequent detection of\nthe weaker but longer lasting IR signal would allow an identification of the\nmerger event. We briefly discuss the implications of our results to the recent\ndetection of an nIR transient accompanying GRB 130603B.\n