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

TL;DR

This paper models the long-term electromagnetic signals from neutron star mergers, predicting faint IR and UV/optical transients powered by radioactive decay and neutrino-driven winds, with implications for gravitational wave follow-up.

Contribution

It provides detailed 3D hydrodynamic simulations of merger ejecta to predict electromagnetic transient light curves, incorporating updated opacities and considering different ejecta components.

Findings

IR peak 2-4 days post-merger with luminosity ~2×10^{40} erg/s

UV/optical transient peaks ~6 hours after merger with luminosity ~10^{41} erg/s

IR signals are weak but detectable with deep follow-up observations

Abstract

We use 3D hydrodynamic simulations of the long-term evolution of neutron star merger ejecta to predict the light curves of electromagnetic transients that are powered by the decay of freshly produced r-process nuclei. For the dynamic ejecta that are launched by tidal and hydrodynamic interaction, we adopt grey opacities of 10 cm$^2$/g, as suggested by recent studies. For our reference case of a 1.3-1.4 $M_\odot$ merger, we find a broad IR peak 2-4 d after the merger. The peak luminosity is $\approx 2\times 10^{40}$ erg/s for an average orientation, but increased by up to a factor of 4 for more favourable binary parameters and viewing angles. These signals are rather weak and hardly detectable within the large error box (~100 deg$^2$) of a gravitational wave trigger. A second electromagnetic transient results from neutrino-driven winds. These winds produce `weak' r-process material with $50 < A < 130$ and abundance patterns that vary substantially between different merger cases. For an adopted opacity of 1 cm$^2$/g, the resulting transients peak in the UV/optical about 6 h after the merger with a luminosity of $\approx 10^{41}$ erg/s (for a wind of 0.01 $M_\odot$) These signals are marginally detectable in deep follow-up searches (e.g. using Hypersuprime camera on Subaru). A subsequent detection of the weaker but longer lasting IR signal would allow an identification of the merger event. We briefly discuss the implications of our results to the recent detection of an nIR transient accompanying GRB 130603B.