Neutron Star Binary Coalescences: Angular Momentum Threshold Against a Prompt Collapse

We performed fully general relativistic simulations of neutron star binary coalescences to investigate how much angular momentum is needed to support the merged system against a prompt gravitational collapse. For a polytropic equation equation of state with the polytropic index in the range of $\Gamma = 2$ to $1.8$, we find that the angular momentum threshold for an equal mass system can be described quite accurately by a simple formula $L/M2=M_{ADM}/M_{ADM}^{max}$, where $L$ is the total angular momentum of the binary neutron star system, $M$ is the total ADM mass of the system, $M_{ADM}$ is the ADM mass of the neutron star in isolation, $M_{ADM}^{max}$ is the maximum ADM mass of a single neutron star with the same equation of state.