Constraint on the maximum mass of neutron stars using GW170817 event

We revisit the constraint on the maximum mass of cold spherical neutron stars coming from the observational results of GW170817. We develop a new framework for the analysis by employing both energy and angular momentum conservation laws as well as solid results of latest numerical-relativity simulations and of neutron stars in equilibrium. The new analysis shows that the maximum mass of cold spherical neutron stars can be only weakly constrained as M_{\rm max} \alt 2.3M_\odot. Our present result illustrates that the merger remnant neutron star at the onset of collapse to a black hole is not necessarily rapidly rotating and shows that we have to take into account the angular momentum conservation law to impose the constraint on the maximum mass of neutron stars.Comment: 14 pages, 5 figures, matches the version accepted by PRD for publicatio

Mass Ejection from the Remnant of a Binary Neutron Star Merger: Viscous-Radiation Hydrodynamics Study

We perform long-term general relativistic neutrino radiation hydrodynamics simulations (in axisymmetry) for a massive neutron star (MNS) surrounded by a torus, which is a canonical remnant formed after the binary neutron star merger. We take into account the effects of viscosity, which is likely to arise in the merger remnant due to magnetohydrodynamical turbulence. As the initial condition, we employ the azimuthally averaged data of the MNS-torus system derived in a three-dimensional, numerical-relativity simulation for the binary neutron star merger. The viscous effect plays key roles for the remnant evolution and mass ejection from it in two phases of the evolution. In the first $t\lesssim10$ ms, a differential rotation state of the MNS is changed to a rigidly rotating state, and as a result, a sound wave, which subsequently becomes a shock wave, is formed in the vicinity of the MNS due to the variation of the quasi-equilibrium state of the MNS. The shock wave induces significant mass ejection of mass $\sim(0.5-2.0)\times 10^{-2}M_\odot$ for the alpha viscosity parameter of $0.01-0.04$. For the longer-term evolution with $\sim 0.1-10$ s, a significant fraction of the torus material is ejected. The ejecta mass is likely to be of order $10^{-2}M_\odot$, so that the total mass of the viscosity-driven ejecta could dominate that of the dynamical ejecta of mass $\lesssim 10^{-2}M_\odot$. The electron fraction, $Y_e$, of the ejecta is always high enough ($Y_e\gtrsim0.25$) that this post-merger ejecta is lanthanide-poor; hence, the opacity of the ejecta is likely to be $\sim 10-100$ times lower than that of the dynamical ejecta. This indicates that the electromagnetic signal from the ejecta would be rapidly evolving, bright, and blue if it is observed from a small viewing angle ($\lesssim 45^\circ$) for which the effect of the dynamical ejecta is minor.Comment: 21 pages, 18 figures, accepted for publication in Ap

A Monte-Carlo based relativistic radiation hydrodynamics code with a higher-order scheme

We develop a new relativistic radiation hydrodynamics code based on the Monte-Carlo algorithm. In this code, we implement a new scheme to achieve the second-order accuracy in time in the limit of a large packet number for solving the interaction between matter and radiation. This higher-order time integration scheme is implemented in the manner to guarantee the energy-momentum conservation to the precision of the geodesic integrator. The spatial dependence of radiative processes, such as the packet propagation, emission, absorption, and scattering, are also taken into account up to the second-order accuracy. We validate our code by solving various test-problems following the previous studies; one-zone thermalization, dynamical diffusion, radiation dragging, radiation mediated shock-tube, shock-tube in the optically thick limit, and Eddington limit problems. We show that our code reproduces physically appropriate results with reasonable accuracy and also demonstrate that the second-order accuracy in time and space is indeed achieved with our implementation for one-zone and one-dimensional problems.Comment: 25 pages, 10 figures, submitted to PR

Properties of neutrino transfer in a deformed remnant of neutron star merger

We study properties of neutrino transfer in a remnant of neutron star merger, consisting of a massive neutron star and a surrounding torus. We perform numerical simulations of the neutrino transfer by solving the Boltzmann equation with momentum-space angles and energies of neutrinos for snapshots of the merger remnant having elongated shapes. The evaluation of the neutrino distributions in the multi-dimensions enable us to provide the detailed information of angle and energy spectra and neutrino reaction rates. We demonstrate features of asymmetric neutrino fluxes from the deformed remnant and investigate the neutrino emission region by determining the neutrinosphere for each energy. We examine the emission and absorption of neutrinos to identify important ingredients of heating rates through neutrino irradiation. We show that the contributions of $\mu$- and $\tau$-types neutrinos are important for the heating in the region above the massive neutron star. We also examine the angle moments and the Eddington tensor calculated directly by the neutrino distribution functions and compare them with those obtained by a moment closure approach, which is often used in the study of neutrino-radiation hydrodynamics. We show that the components of the Eddington tensor have non-monotonic behaviors and the approximation of the closure relation may become inaccurate for high energy neutrinos, whose fluxes are highly aspherical due to the extended merger remnant.Comment: 28 pages, 33 figures, revised version accepted for publication in Ap