Magnetohydrodynamics of Neutrino-Cooled Accretion Tori around a Rotating Black Hole in General Relativity

Abstract

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 $\sim 0.1$--0.4$M_{\odot }$ around a black hole of mass $M=4M_{\odot }$ 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 $\approx 60$ 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 $\alpha$-particles, black body radiation, and a relativistic Fermi gas (neutrinos, electrons, and positrons). Neutrino-emission processes, such as $e^{\pm }$ capture onto free nucleons, $e^{\pm }$ 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 ${\dot{M}}_{\ast }\sim 1$--$10M_{\odot }$/s, and the shock heating increases the temperature to $\sim 10^{11}$ K. This results in a maximum neutrino emission rate of $L_{\nu }=$ several $\times 10^{53}$ ergs/s and a conversion efficiency $L_{\nu }/{\dot{M}}_{\ast }{c}^2$ on the order of a few percent for tori with mass $M_{\mathrm{t}}\approx 0.1$--0.4$M_{\odot }$ and for moderately high black hole spins.

Shibata, M., Sekiguchi, Y., Takahashi, R., Progress of Theoretical Physics, 118, 257 (2007)

References in corpus (1)

• Gamma-Ray Bursts: Progress, Problems & Prospects

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