Gravitational-wave observation together with a large number of electromagnetic observations shows that the source of the latest gravitational-wave event , GW170817 , detected primarily by advanced LIGO , is the merger of a binary neutron star .
We attempt to interpret this observational event based on our results of numerical-relativity simulations performed so far paying particular attention to the optical and infra-red observations .
We finally reach a conclusion that this event is described consistently by the presence of a long-lived hypermassive or supramassive neutron star as the merger remnant , because ( i ) significant contamination by lanthanide elements along our line of sight to this source can be avoided by the strong neutrino irradiation from it and ( ii ) it could play a crucial role to produce an ejecta component of appreciable mass with fast motion in the post-merger phase .
We also point out that ( I ) the neutron-star equation of state has to be sufficiently stiff ( i.e. , the maximum mass of cold spherical neutron stars , M _ { max } , has to be appreciably higher than 2 M _ { \odot } ) in order that a long-lived massive neutron star can be formed as the merger remnant for the binary systems of GW170817 , for which the initial total mass is \agt 2.73 M _ { \odot } and ( II ) no detection of relativistic optical counterpart suggests a not-extremely high value of M _ { max } approximately as 2.15– 2.25 M _ { \odot } .