A low-mass binary neutron star: long-term ejecta evolution and kilonovae with weak blue emission

TL;DR

This study models the long-term ejecta evolution from a binary neutron star merger with a long-lived remnant NS, revealing unique light curve features that could indicate the remnant's nature and challenge previous interpretations of GW170817.

Contribution

It provides the first detailed hydrodynamics and radiative transfer simulation of ejecta from a long-lived neutron star remnant, highlighting the impact on kilonova light curves and remnant fate inference.

Findings

Infrared emission can be brighter than optical despite large ejecta mass.

Ejecta morphology causes preferential photon diffusion toward the equator.

Bright optical emission suggests the remnant likely collapsed to a black hole within 0.1 s.

Abstract

We study the long-term evolution of ejecta formed in a binary neutron star (BNS) merger that results in a long-lived remnant NS by performing a hydrodynamics simulation with the outflow data of a numerical relativity simulation as the initial condition. At the homologously expanding phase, the total ejecta mass reaches $\approx0.1\,M_\odot$ with an average velocity of $\approx0.1\,c$ and lanthanide fraction of $\approx 0.005$. We further perform the radiative transfer simulation employing the obtained ejecta profile. We find that, contrary to a naive expectation from the large ejecta mass and low lanthanide fraction, the optical emission is not as bright as that in GW170817/AT2017gfo, while the infrared emission can be brighter. This light curve property is attributed to preferential diffusion of photons toward the equatorial direction due to the prolate ejecta morphology, large opacity contribution of Zr, Y, and lanthanides, and low specific heating rate of the ejecta. Our results suggest that these light curve features could be used as an indicator for the presence of a long-lived remnant NS. We also found that the bright optical emission broadly consistent with GW170817/AT2017gfo is realized for the case that the high-velocity ejecta components in the polar region are suppressed. These results suggest that the remnant in GW170817/AT2017gfo is unlikely to be a long-lived NS, but might have collapsed to a black hole within ${\cal O}(0.1)$ s.