CONSTRAINTS ON THE HIGH-DENSITY EQUATION OF STATE FROM THE GRAVITATIONAL-WAVE SIGNAL OF NEUTRON STAR MERGERS

We present a constraint on neutron star radii, which is derived from a multi-messenger interpretation of GW170817, the first unambiguously detected neutron star merger event. The optical and infrared emission of the electromagnetic counterpart was relatively bright compared to simulation results. We argue that the remnant of the merger did not undergo a prompt collapse to a black hole because this results in relatively dim electromagnetic transients. If this interpretation is correct, neutron star radii cannot be too small to prevent direct collapse. We find that the radius of a non-rotating neutron star with a mass of 1.6 M-circle dot should be larger than about 10.7 km excluding very soft nuclear matter. We emphasize the potential of future multi-messenger observations to which the same arguments and procedures can be applied, and which can then yield more stringent radius limits. Furthermore, a prompt collapse event can place an upper bound on the maximum mass of nonrotating neutron stars.