  Massive (hypermassive and supramassive) neutron stars are likely to be often
formed after the merger of binary neutron stars. We explore the evolution
process of the remnant massive neutron stars and gravitational waves emitted by
them, based on numerical-relativity simulations for binary neutron star mergers
employing a variety of equations of state and choosing a plausible range of the
neutron-star mass of binaries. We show that the lifetime of remnant
hypermassive neutron stars depends strongly on the total binary mass and also
on the equations of state. Gravitational waves emitted by the remnant massive
neutron stars universally have a quasiperiodic nature of an approximately
constant frequency although the frequency varies with time. We also show that
the frequency and time-variation feature of gravitational waves depend strongly
on the equations of state. We derive a fitting formula for the quasiperiodic
gravitational waveforms, which may be used for the data analysis of a
gravitational-wave signal.
