Magnetohydrodynamics of Neutrino-Cooled Accretion Tori around a Rotating Black Hole in General Relativity(Astrophysics and Relativity)
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概要
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We present our first numerical results of axisymmetric magnetohydrodynamic simulations for neutrino-cooled accretion tori around rotating black holes in general relativity. We consider tori of mass 〜0.1-0.4M_⊙ around a black hole of mass M=4M_⊙ and spin a=0-0.9M; such systems are candidates for the central engines of gamma-ray bursts (GRBs) formed after the collapse of massive rotating stellar cores and the merger of a black hole and a neutron star. In this paper, we consider the short-term evolution of a torus for a duration of ≈ 60 ms, focusing on short-hard GRBs. Simulations were performed with a plausible microphysical equation of state that takes into account neutronization, the nuclear statistical equilibrium of a gas of free nucleons and α-particles, black body radiation, and a relativistic Fermi gas (neutrinos, electrons, and positrons). Neutrino-emission processes, such as e^± capture onto free nucleons, e^± pair annihilation, plasmon decay, and nucleon-nucleon bremsstrahlung are taken into account as cooling processes. Magnetic braking and the magnetorotational instability in the accretion tori play a role in angular momentum redistribution, which causes turbulent motion, resultant shock heating, and mass accretion onto the black hole. The mass accretion rate is found to be M_*〜1-10M_⊙/s, and the shock heating increases the temperature to 〜10^<11>K. This results in a maximum neutrino emission rate of L_ν=several×10^<53> ergs/s and a conversion efficiency L_ν/M_*c^2 on the order of a few percent for tori with mass M_t≈0.1-0.4M_⊙ and for moderately high black hole spins. These results are similar to previous results in which the phenomenological α-viscosity prescription with the α-parameter of α_v=0.01-0.1 is used. It is also found that the neutrino luminosity can be enhanced by the black hole spin, in particular for large spins, i.e., a≳0.75M; if the accretion flow is optically thin with respect to neutrinos, the conversion efficiency may be ≳10% for a≳0.9M. Angular momentum transport, and the resulting shock heating caused by magnetic stress induce time-varying neutrino luminosity, which is a favorable property for explaining the variability of the luminosity curve of GRBs.
- 2007-08-25
著者
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Takahashi Rohta
Graduate School Of Arts And Sciences University Of Tokyo
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Shibata Masaru
Graduate School Of Arts And Sciences University Of Tokyo
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SEKIGUCHI Yuichiro
Graduate School of Arts and Sciences, University of Tokyo
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SEKIGUCHI Yu-ichiro
Graduate School of Arts and Sciences, University of Tokyo
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Sekiguchi Yu-ichiro
Graduate School Of Arts And Sciences University Of Tokyo
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SHIBATA Masaru
Yukawa Institute for Theoretical Physics, Kyoto University
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TAKAHASHI Rohta
Graduate School of Arts and Sciences, University of Tokyo
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