Radiation Magnetohydrodynamics for Black Hole-Torus System in Full General Relativity: A Step toward Physical Simulation

TL;DR

This paper presents the first full general relativistic radiation-magnetohydrodynamic simulation of a black hole-torus system, revealing high neutrino luminosities and potential gamma-ray burst mechanisms post-neutron star merger.

Contribution

It introduces a novel simulation framework for black hole-torus systems in full general relativity with neutrino transport, advancing understanding of post-merger astrophysical phenomena.

Findings

High temperature in the torus enhances neutrino luminosity.

Neutrino emission is primarily outward along the rotational axis.

Neutrino luminosity can surpass electromagnetic emission for certain black hole masses.

Abstract

A radiation-magnetohydrodynamic simulation for the black hole-torus system is performed in the framework of full general relativity for the first time. A truncated moment formalism is employed for a general relativistic neutrino radiation transport. Several systems in which the black hole mass is $M_{\rm BH}=3$ or $6M_{\odot}$, the black hole spin is zero, and the torus mass is $\approx 0.14$--$0.38M_{\odot}$ are evolved as models of the remnant formed after the merger of binary neutron stars or black hole-neutron star binaries. The equation of state and microphysics for the high-density and high-temperature matter are phenomenologically taken into account in a semi-quantitative manner. It is found that the temperature in the inner region of the torus reaches $\agt 10$ MeV which enhances a high luminosity of neutrinos $\sim 10^{51}$ ergs/s for $M_{\rm BH}=6M_{\odot}$ and $\sim 10^{52}$ ergs/s for $M_{\rm BH}=3M_{\odot}$. It is shown that neutrinos are likely to be emitted primarily toward the outward direction in the vicinity of the rotational axis and their energy density may be high enough to launch a low-energy short gamma-ray burst via the neutrino-antineutrino pair-annihilation process with the total energy deposition $\sim 10^{47}$--$10^{49}$ ergs. It is also shown in our model that for $M_{\rm BH}=3M_{\odot}$, the neutrino luminosity is larger than the electromagnetic luminosity while for $M_{\rm BH}=6M_{\odot}$, the neutrino luminosity is comparable to or slightly smaller than the electromagnetic luminosity.