General-relativistic neutrino-radiation magnetohydrodynamics simulation of black hole-neutron star mergers for seconds

Abstract

Seconds-long numerical-relativity simulations for black hole-neutron star mergers are performed for the first time to obtain a self-consistent picture of the merger and post-merger evolution processes. To investigate the case that tidal disruption takes place, we choose the initial mass of the black hole to be 5.4M or 8.1M with the dimensionless spin of 0.75. The neutron-star mass is fixed to be 1.35M . We find that after the tidal disruption, dynamical mass ejection takes place spending 10 ms together with the formation of a massive accretion disk. Subsequently, the magnetic field in the disk is amplified by the magnetic winding and magnetorotational instability, establishing a turbulent state and inducing the angular momentum transport. The post-merger mass ejection by the magnetically-induced viscous effect sets in at ∼ 300-500 ms after the tidal disruption, at which the neutrino luminosity drops below ∼ 10 51.5 erg/s, and continues for several hundreds ms. A magnetosphere near the rotational axis of the black hole is developed after the matter and magnetic flux fall into the black hole from the accretion disk, and high-intensity Poynting flux generation sets in at a few hundreds ms after the tidal disruption. The intensity of the Poynting flux becomes low after the significant post-merger mass ejection, because the opening angle of the magnetosphere increases. The lifetime for the stage with the strong Poynting flux is 1-2 s, which agrees with the typical duration of short-hard gamma-ray bursts.

Fast Neutrino Conversion in Hydrodynamic Simulations of Neutrino-Cooled Accretion Disks

“…We constrain ourselves here to the case of axisymmetric viscous models. However, we expect that the main result of this exercise (namely that ELN crossings appear in large regions throughout the disk) would also be obtained when using 3D MHD models, which exhibit a similar structure as viscous disks, at least in a time-averaged sense [23,36,37,42].…”

“…Disks accreting material onto stellar mass black holes (BHs) with a rate higher than ∼ 10 −3 -10 −2 M s −1 , which are formed after the merger of two neutron stars (NSs) or a NS with a BH [1][2][3][4][5][6][7] or during the collapse of a fast rotating star [8][9][10][11][12], are called neutrino-cooled disks, or neutrino-dominated accretion flows (NDAFs), because weak interactions take place on timescales comparable to the dynamical timescales, enabling neutrinos to radiate away most of the heat generated in the turbulent flow [13][14][15][16][17][18][19][20][21][22][23][24][25]. The high neutrino fluxes, and therefore high neutrino pair-annihilation rates, produced in these disks have been suggested to power ultrarelativistic jets of short gamma-ray bursts [26][27][28].…”

Implementation of advanced Riemann solvers in a neutrino-radiation magnetohydrodynamics code in numerical relativity and its application to a binary neutron star merger

“…Furthermore, very recently we performed neutrinoradiation magnetohydrodynamics simulations of black hole-neutron star mergers in numerical relativity [55]. We found post-merger mass ejection due to magnetorotational instability-driven turbulence and the launch of a Poynting flux-dominated outflow.…”

“…The post-merger mass ejection and the Poynting-flux dominated outflow sets in at several 100 ms after the merger and lasts for 1-2 seconds after the merger. These timescales are determined by the strength of the effective viscosity associated with both magnetorotational-instability turbulence and neutrino cooling [55].…”

Long-term 3D-MHD Simulations of Black Hole Accretion Disks formed in Neutron Star Mergers

“…Capturing this instability requires three-dimensional (3D) magnetohydrodynamic (MHD) simulations, which are computationally expensive given the long evolutionary times in the problem. Consequently, only a handful of simulations of BH accretion disks in 3D general-relativistic (GR) MHD with some form of neutrino physics have been conducted thus far (Hossein Nouri et al 2018;Siegel & Metzger 2017Fernández et al 2019a;Miller et al 2019;Christie et al 2019;Just et al 2022;Murguia-Berthier et al 2021;Hayashi et al 2021) following earlier work in axisymmetry (Shibata et al 2007;Shibata & Sekiguchi 2012;Janiuk et al 2013;Janiuk 2017).…”