A lower bound on the maximum mass if the secondary in GW190814 was once a rapidly spinning neutron star

Abstract

The recent detection of GW190814 featured the merger of a binary with a primary having a mass of ∼23 M_⊙ and a secondary with a mass of ∼2.6 M_⊙⁠. While the primary was most likely a black hole, the secondary could be interpreted as either the lightest black hole or the most massive neutron star ever observed, but also as the indication of a novel class of exotic compact objects. We here argue that although the secondary in GW190814 is most likely a black hole at merger, it needs not be an ab-initio black hole nor an exotic object. Rather, based on our current understanding of the nuclear-matter equation of state, it can be a rapidly rotating neutron star that collapsed to a rotating black hole at some point before merger. Using universal relations connecting the masses and spins of uniformly rotating neutron stars, we estimate the spin, 0.49^(+0.08)_(−0.05) ≲ χ ≲ 0.68^(+0.11)_(−0.05)⁠, of the secondary – a quantity not constrained so far by the detection – and a novel strict lower bound on the maximum mass, M_(TOV) > 2.08^(+0.04)_(−0.04) M_⊙ and an optimal bound of M_(TOV) > 2.15^(+0.04)_(−0.04) M_⊙⁠, of non-rotating neutron stars, consistent with recent observations of a very massive pulsar. The new lower bound also remains valid even in the less likely scenario in which the secondary neutron star never collapsed to a black hole.

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