Systematics of prompt black-hole formation in neutron star mergers.

This study addresses the collapse behavior of neutron star (NS) mergers expressed through the binary threshold mass M_thres for prompt black hole (BH) formation, which we determine by relativistic hydrodynamical simulations for a set of 40 equation of state (EoS) models. M_thres 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=M_1/M_2 and assemble different fits, which may be applicable in different situations. We find fit formulae for M_thres 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 for some extreme cases the threshold mass of asymmetric mergers is equal or smaller than the one of equal-mass binaries. Generally, the impact of the binary mass asymmetry on M_thres becomes stronger with more extreme mass ratios, while M_thres is approximately constant for small deviations from q=1, i.e. for 0.85<=q<=1. We also describe that a phase transition to deconfined quark matter can leave a characteristic imprint on the collapse behavior of NS mergers. The presence of quark matter can reduce the threshold mass such that it is particularly small relative to the combined tidal deformability Lambda_thres of the system with M_thres. Hence, a combined measurement or a constraint on both quantities can indicate the onset of quark deconfinement. We point out new univariate relations between M_thres and stellar properties of high-mass NSs, which can be employed for direct EoS constraints or consistency checks. (abridged)