Evidence for a maximum mass cut-off in the neutron star mass distribution and constraints on the equation of state

Justin Alsing, Hector O. Silva, Emanuele Berti

Published 2017-09-22Version 1

We infer the mass distribution of neutron stars in binary systems using a flexible Gaussian mixture model and use Bayesian model selection to explore evidence for multi-modality and a sharp cut-off in the mass distribution. We find overwhelming evidence for a bimodal distribution, in agreement with previous literature, and report for the first time positive evidence for a sharp cut-off at a maximum neutron star mass. We measure the maximum mass to be $2.06M_\odot < m_\mathrm{max} < 2.24M_\odot$ (68\%), $2.0M_\odot < m_\mathrm{max}< 2.5M_\odot$ (90\%), where this constraint is robust against the choice of model for the mass distribution and to removing the most extreme (highest mass) neutron stars from the dataset. If this sharp cut-off is interpreted as the maximum stable neutron star mass allowed by the equation of state of dense matter, our measurement puts tight constraints on the equation of state. For a set of realistic equations of state that support $>2M_\odot$ neutron stars, our inference of $m_\mathrm{max}$ is able to distinguish between models at odds ratios of up to $15:1$, whilst under a flexible piecewise polytropic equation of state model our maximum mass measurement improves constraints on the pressure at $3-7\times$ the nuclear saturation density by $\sim 35-50\%$ compared to simply requiring $m_\mathrm{max} > 2M_\odot$. We obtain a lower bound on the maximum sound speed attained inside the neutron star of $c_s^\mathrm{max} > 0.64c$ (99.8\%), ruling out $c_s^\mathrm{max}<c/\sqrt{3}$ at high significance. Our constraints on the equation of state strengthen the case for neutron star-neutron star mergers as the primary source of short gamma-ray bursts.