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It has been estimated that a significant proportion of binary neutron starmerger events produce long-lived massive remnants supported by differentialrotation and subject to rotational instabilities. To examine formation andoscillation of rapidly rotating neutron stars (NS) after merger, we present anexploratory study of fully general-relativistic hydrodynamic simulations usingthe public code Einstein Toolkit. The attention is focused on qualitativeaspects of long-term postmerger evolution. As simplified test models, we use amoderately stiff Gamma=2 ideal-fluid equation of state and unmagnetizedirrotational equal-mass binaries with three masses well below the threshold forprompt collapse. Our high resolution simulations generate postmerger "ringdown"gravitational wave (GW) signals of 170 ms, sustained by rotating massive NSremnants without collapsing to black holes. We observe that the high-densitydouble-core structure inside the remnants gradually turns into aquasi-axisymmetric toroidal shape. It oscillates in a quasi-periodic manner andshrinks in size due to gravitational radiation. In the GW spectrograms,dominant double peaks persist throughout the postmerger simulations and slowlydrift to higher frequencies. A new low-frequency peak emerges at about 100 msafter merger, owing to the growth of GW-driven unstable oscillation modes. Thelong-term effect of grid resolution is also investigated using the same initialmodel. Moreover, we comment on physical conditions that are favorable for thetransient toroidal configuration to form, and discuss implication of ourfindings on future GW observation targeting rapidly rotating NSs.

Long-Term Postmerger Simulations of Binary Neutron Star Coalescence: Formation of Toroidal Remnants and Gravitational Wave Afterglow