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Abstract

We revisit the lower bound on binary tidal deformability tilde{Lambda}
imposed by a luminous kilonova/macronova, AT 2017gfo, by numerical-relativity
simulations of models consistent with gravitational waves from the
binary-neutron-star merger, GW170817. Contrary to the claim made in the
literature, we find that binaries with tilde{Lambda}<~400 can explain the
luminosity of AT 2017gfo as far as moderate mass ejection from the remnant is
assumed as also done in the previous work. The reason is that the maximum mass
of a neutron star is not strongly correlated with tidal deformability of
neutron stars with typical mass of ~1.4M_sun. If the maximum mass is so large
that the binary does not collapse to a black hole immediately after merger, the
mass of the ejecta can be sufficiently large irrespective of the binary tidal
deformability. We explicitly present models of binary mergers with
tilde{Lambda} down to 242 that satisfy the requirement on the mass of the
ejecta from the luminosity of AT 2017gfo. We further find that the luminosity
of AT 2017gfo could even be explained by models that do not experience bounce
after merger. We conclude that the luminosity of AT 2017gfo is not very useful
to constrain the binary tidal deformability. Accurate estimation of the mass
ratio will be necessary to put a lower bound using electromagnetic counterparts
in the future. We also caution that merger simulations performed employing a
limited class of tabulated equations of state could be severely biased due to
the lack of generality.