2017ApJ...850L..19M

We combine electromagnetic (EM) and gravitational-wave (GW) information on the binary neutron star (NS) merger GW170817 in order to constrain the radii R_ns and maximum mass M_{max} of NSs. GW170817 was followed by a range of EM counterparts, including a weak gamma-ray burst (GRB), kilonova (KN) emission from the radioactive decay of the merger ejecta, and X-ray/radio emission consistent with being the synchrotron afterglow of a more powerful off-axis jet. The type of compact remnant produced in the immediate merger aftermath, and its predicted EM signal, depend sensitively on the high-density NS equation of state (EOS). For a soft EOS that supports a low M_{max}, the merger undergoes a prompt collapse accompanied by a small quantity of shock-heated or disk-wind ejecta, inconsistent with the large quantity 10^–2 M_☉ of lanthanide-free ejecta inferred from the KN. On the other hand, if M_{max} is sufficiently large, then the merger product is a rapidly rotating supramassive NS (SMNS), which must spin down before collapsing into a black hole. A fraction of the enormous rotational energy necessarily released by the SMNS during this process is transferred to the ejecta, either into the GRB jet (energy E_GRB) or the KN ejecta (energy E_ej), also inconsistent with observations. By combining the total binary mass of GW170817 inferred from the GW signal with conservative upper limits on E_GRB and E_ej from EM observations, we constrain the likelihood probability of a wide range of previously allowed EOSs. These two constraints delineate an allowed region of the M_{max}–R_ns parameter space, which, once marginalized over NS radius, places an upper limit of M_{max} 2.17 M_☉ (90%), which is tighter or arguably less model-dependent than other current constraints.