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, $0.49_{-0.05}^{+0.08} \lesssim \chi \lesssim 0.68_{-0.05}^{+0.11}$, of the secondary – a quantity not constrained so far by the detection – and a novel strict lower bound on the maximum mass, $M_{_{\mathrm{TOV}}}> 2.08^{+0.04}_{-0.04}\, \, M_{\odot }$ and an optimal bound of $M_{_{\mathrm{TOV}}}> 2.15^{+0.04}_{-0.04}\, \, M_{\odot }$, of nonrotating 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.

Progress in Constraining Nuclear Symmetry Energy Using Neutron Star Observables Since GW170817

“…On the other hand, the possibility for GW190814's secondary as a neutron star can be accomplished by: (1) choosing/constructing stiff EOSs having a maximum mass larger than 2.5 M [76,[141][142][143][144][145][146][147][148][149][150]; (2) considering the effects of fast rotations, which can increase the maximum mass by about 20% when a star rotates at the Kepler frequency (the maximum frequency at which the gravitational attraction is still sufficient to keep matter bound to the pulsar surface) [42,43,139,140,145,[151][152][153][154][155][156][157] ; (3) considering other effects/models that can modify the maximum mass of a neutron star, such as the magnetic field [147], twin star [158], two families of compact stars [142], finite temperature [153], antikaon condensation [159], net electric charge [144], etc.…”

Multimessenger Binary Mergers Containing Neutron Stars: Gravitational Waves, Jets, and γ-Ray Bursts

“…Assuming that this was the case, it is possible to place strong constraints on the maximum mass of a cold spherical NS and its EOS (Margalit and Metzger, 2017;Shibata et al, 2017Shibata et al, , 2019Rezzolla et al, 2018;Ruiz et al, 2018a). These constraints could also provide an explanation for the unidentified 2.6 M ⊙ compact object in GW190814 as a rotating or even a non-rotating NS (Most et al, 2020;Tsokaros et al, 2020a). From a different point of view, the absence of a prompt collapse scenario and the large ejecta mass also puts constraints on NS radii or, equivalently, their tidal deformability (Bauswein et al, 2017;Radice et al, 2018).…”

The energy budget of the transition of a neutron star into the third family branch

“…Colorful regions in the mass–radius diagram include state‐of‐the art constraints from multimessenger astronomy observations as described in the Figure 3 caption. In particular, the derivations of a possible lower bound on the maximum compact star by Rezzolla et al (2018) and by Most et al (2020) have a strong dependence on the assumptions on the components and remnants of the mergers of GW170817 and GW190814; therefore, we do not strictly rule out EoS models leading to maximum masses above 2.04 M ⊙ . Interestingly, recent X‐ray observations of prolonged emissions of the remnant of GW170817 suggest the possibility of the formation of a compact star in lieu of a black hole (Troja et al 2020).…”

“…The latter is valid under the assumption that the merged compact stars collapsed into a black hole. The horizontal band delimited by dashed lines that is centered around 2.08 M ⊙ corresponds to a lower boundary on the maximum neutron star mass derived from the event GW190814 signal under the assumption that one of the components of the merger was a fast rotating neutron star (Most et al 2020). The upper left gray region is inaccessible to neutron stars due to causality violation on the equation of state…”