The heavier the better: how to constrain mass ratios and spins of high-mass neutron-star mergers

Abstract

The first binary neutron-star merger event, GW170817, and its bright electromagnetic counterpart have provided a remarkable amount of information. By contrast, the second event, GW190425, with $M_{\mathrm{tot}}=3.4_{-0.1}^{+0.3}{M}_{\odot }$ and the lack of an electromagnetic counterpart, has hardly improved our understanding of neutron-star physics. While GW190425 is compatible with a scenario in which the merger has lead to a prompt collapse to a black hole and little ejected matter to power a counterpart, determining the mass ratio and the effective spin $\tilde{\chi }$ of the binary remains difficult. This is because gravitational waveforms cannot yet well constrain the component spins of the binary. However, since the mass of GW190425 is significantly larger than the maximum mass for nonrotating neutron stars, $M_{{}_{\mathrm{TOV}}}$, the mass ratio $q$ cannot be too small, as the heavier star would not be gravitationally stable. Making use of universal relations and a large number of equations of state, we provide limits in the $(\tilde{\chi },q)$ plane for GW190425, namely: $q_{\min }\ge 0.38$ and ${\tilde{\chi }}_{\max }\le 0.20$, assuming $M_{\mathrm{tot}}\simeq 3.4M_{\odot }$. Finally, we show how future observations of high-mass binaries can provide a lower bound on $M_{{}_{\mathrm{TOV}}}$.

6 pages, 4 figures, Accepted version

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• Bayesian analysis of neutron-star properties with parameterized equations of state: the role of the likelihood functions
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• On accretion disks formed in MHD simulations of black hole-neutron star mergers with accurate microphysics
• Population properties and multimessenger prospects of neutron star-black hole mergers following GWTC-3
• Impact of extreme spins and mass ratios on the post-merger observables of high-mass binary neutron stars
• Fast ejecta as a potential way to distinguish black holes from neutron stars in high-mass gravitational-wave events
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• Mass and radius of the most massive neutron star: The probe of the equation of state and perturbative QCD