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 ∼ 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 nucleonnucleon 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 beṀ * ∼ 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 /Ṁ * 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. at East Tennessee State University on May 29, 2015 http://ptp.oxfordjournals.org/ Downloaded from at East Tennessee State University on May 29, 2015 http://ptp.oxfordjournals.org/ Downloaded from We numerically solved Eq. (2 . 33) in the same manner as Eq. (2 . 26). at East Tennessee State University on May 29, 2015http://ptp.oxfordjournals.org/ Downloaded from * ) In the realistic EOSs for high-density matter with ρ ≥ 10 7 g/cm 3 , ε is determined primarily by the radiation pressure for the high-temperature case, whereas it is determined by the electrondegenerate pressure for the low-temperature case. This implies that ε is proportional to T Downloaded from * ) As a result of the anisotropic emission, angular momentum is dissipated, but the magnitude of this effect is much smaller than the loss associated with the infal...

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