Systematics of prompt blackhole formation in neutron star mergers
Abstract
This study addresses the collapse behavior of neutron star (NS) mergers expressed through the binary threshold mass M_{thres} for prompt blackhole (BH) formation, which we determine by relativistic hydrodynamical simulations for a set of 40 equation of state (EOS) models of NS matter. M_{thres} can be well described by various fit formulas involving stellar parameters of nonrotating NSs, which are employed to characterize the EOS models. Using these relations we compute which constraints on NS radii and the tidal deformability are set by current and future merger detections that reveal information about the merger product. We systematically investigate the impact of the binary mass ratio q =M_{1}/M_{2} and assemble various fits, which make different assumptions about apriori knowledge. This includes fit formulas for M_{thres} for a fixed mass ratio or a range of q if this parameter is not known very well. Also, we construct relations describing the threshold to prompt collapse for different classes of candidate EOSs, which for instance do or do not include models with a phase transition to quark matter. In particular, we find fit formulas 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 equalmass 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 . The magnitude of the reduction of M_{thres} with the binary mass asymmetry follows a systematic EOS dependence. We also describe in more detail that a phase transition to deconfined quark matter can leave a characteristic imprint on the collapse behavior of NS mergers. The occurrence of quark matter can reduce the stability of the remnant and thus the threshold mass relative to a purely hadronic reference model. Comparing specifically the threshold mass and the combined tidal deformability Λ_{∼thres} of a system with M_{thres} can yield peculiar combinations of those two quantities, where M_{thres} is particularly small in relation to Λ_{∼thres}. Since no purely hadronic EOS can yield such a combination of M_{thres} and Λ_{∼thres}, a combined measurement or a constraint on both quantities can indicate the onset of quark deconfinement. Finally, we point out new univariate relations between M_{thres} and stellar properties of highmass NSs, which can be employed for direct EOS constraints or consistency checks in combination with other measurements.
 Publication:

Physical Review D
 Pub Date:
 June 2021
 DOI:
 10.1103/PhysRevD.103.123004
 arXiv:
 arXiv:2010.04461
 Bibcode:
 2021PhRvD.103l3004B
 Keywords:

 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 35 pages, 13 figures, 14 tables, accepted for publication in Phys. Rev. D