Systematics of prompt black-hole formation in neutron star mergers
Abstract
This study addresses the collapse behavior of neutron star (NS) mergers expressed through the binary threshold mass Mthr for prompt black hole (BH) formation, which we determine by relativistic hydrodynamical simulations for 40 equation of state (EoS) models. Mthr can be well described by various fit formulae involving stellar parameters of nonrotating NSs. Using these relations we compute which constraints on NS radii and the tidal deformability are set by current and future merger detections revealing information about the merger product. We systematically investigate the impact of the binary mass ratio q=M1/M2 and assemble different fits, which make different assumptions about a-priori knowlegde. We find fit formulae for Mthr including an explicit q dependence, which are valid in a broad range of 0.7<=q<=1 and which are nearly as tight as relations for fixed mass ratios. For most EoS models except extreme cases Mthr of asymmetric mergers is equal or smaller than the one of equal-mass binaries. The impact of the binary mass asymmetry on Mthr becomes stronger with more extreme mass ratios, while Mthr is approximately constant for small deviations from q=1. We describe that a phase transition to deconfined quark matter can leave a characteristic imprint on the collapse behavior. The presence of quark matter can reduce the stability of the remnant and thus Mthr relative to a purely hadronic reference model. Comparing the threshold mass and the tidal deformability Lambdathr of a system with Mthr can yield peculiar combinations of those two quantities, where Mthr is particularly small in relation to Lambdathr. Hence, a combined measurement of both quantities can indicate the onset of quark deconfinement. We point out new univariate relations between Mthr and stellar properties of high-mass NSs, which can be employed for direct EoS constraints or consistency checks. (abridged)