| Budget Amount *help |
¥3,400,000 (Direct Cost: ¥3,400,000)
Fiscal Year 2006: ¥1,600,000 (Direct Cost: ¥1,600,000)
Fiscal Year 2005: ¥1,800,000 (Direct Cost: ¥1,800,000)
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| Research Abstract |
Astrophysical systems of two neutron stars are referred to as binary neutron stars. Such binary of close orbits emits gravitational waves, resulting in decreasing the orbital separation to merger. According to statistical studies, the merger happens once a year in a distance of 50 Mpc around our Galaxy. In the last about 15 minutes just before the merger, gravitational waves of more than 10Hz is emitted. Such high frequency gravitational waves are among the most promising sources of gravitational wave detector such as LIGO. Predicting the gravitational waveforms is an issue in theoretical astrophysics.
After merger of binary neutron stars, a black hole or a hypermassive neutron star (HMNS) is formed. If a black hole surrounded by massive, hot, and dense torus is formed, it may be a central engine of gamma-ray burst. Although this hypothesis has been proposed for about 20 years, no one knows if it is correct. It is one of the important issues in astrophysics to clarify whether this scena
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rio is correct or not.
By these two motivations, we have performed numerical simulations in full general relativity for two years. We have reached the following conclusion. (1) A black hole is formed after merger of binary neutron stars if the total mass exceeds a critical mass. It depends on the equation of state of neutron stars ; for plausible stiff nuclear equations of state, the critical mass is 2.6-2.8 solar mass. (2) If the total mass is smaller than the critical mass, an HMNS of elliptical shape is formed. Because of its elliptic shape, the HMNS emits gravitational waves and dissipates angular momentum. As a result of the reduced centrifugal force, the HMNS collapses to a black hole in about 100 ms. (3) Accretion torus is formed around a black hole if the mass ratio of binary neutron stars is not unity. But even for the mass ratio 2/3, the torus mass is about 10% of the solar mass. (4) For the black hole formation, gravitational waves associated with the quasinormal mode ringing are emitted in the last moment.
The amplitude is about 10^<-23> at a distance of 50 Mpc and the frequency is about 7 kHz. It is very difficult to detect such gravitational waves unless the merger happens around our Galaxy by chance. (5) In the case of HMNS formation, quasiperiodic gravitational waves are emitted due to rotation of elliptic HMNS. The effective amplitude is about 10^<-20> at a distance of 50 Mpc and the frequency is about 3 kHz. Such gravitational waves may be detected by the next-generation detectors. (6) After merger of unequal mass binaries, a black hole surrounded by a massive torus is formed. Such system is a candidate of the central engine of gamma-ray bursts. Less
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