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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.