General relativistic viscous hydrodynamics of differentially rotating neutron stars

Employing a simplified version of the Israel-Stewart formalism for general-relativistic shear-viscous hydrodynamics, we perform axisymmetric general-relativistic simulations for a rotating neutron star surrounded by a massive torus, which can be formed from differentially rotating stars. We show that with our choice of a shear-viscous hydrodynamics formalism, the simulations can be stably performed for a long time scale. We also demonstrate that with a possibly high shear-viscous coefficient, not only viscous angular momentum transport works but also an outflow could be driven from a hot envelope around the neutron star for a time scale $\gtrsim 100\,$ms with the ejecta mass $\gtrsim 10^{-2}M_\odot$ which is comparable to the typical mass for dynamical ejecta of binary neutron star mergers. This suggests that massive neutron stars surrounded by a massive torus, which are typical outcomes formed after the merger of binary neutron stars, could be the dominant source for providing neutron-rich ejecta, if the effective shear viscosity is sufficiently high, i.e., if the viscous $\alpha$ parameter is $\gtrsim 10^{-2}$. The present numerical result indicates the importance of a future high-resolution magnetohydrodynamics simulation that is the unique approach to clarify the viscous effect in the merger remnants of binary neutron stars by the first-principle manner.Comment: 16pages, 11pages, PRD accepte

The dynamical mass ejection from binary neutron star mergers: Radiation-hydrodynamics study in general relativity

We perform radiation-hydrodynamics simulations of binary neutron star mergers in numerical relativity on the Japanese "K" supercomputer, taking into account neutrino cooling and heating by an updated leakage-plus-transfer scheme for the first time. Neutron stars are modeled by three modern finite-temperature equations of state (EOS) developed by Hempel and his collaborators. We find that the properties of the dynamical ejecta of the merger such as total mass, average electron fraction, and thermal energy depend strongly on the EOS. Only for a soft EOS (the so-called SFHo), the ejecta mass exceeds $0.01M_{\odot}$. In this case, the distribution of the electron fraction of the ejecta becomes broad due to the shock heating during the merger. These properties are well-suited for the production of the solar-like $r$-process abundance. For the other stiff EOS (DD2 and TM1), for which a long-lived massive neutron star is formed after the merger, the ejecta mass is smaller than $0.01M_{\odot}$, although broad electron-fraction distributions are achieved by the positron capture and the neutrino heating.Comment: 7 pages, 5 figures, accepted to PR

Three dimensional evolution of differentially rotating magnetized neutron stars

We construct a new three-dimensional general relativistic magnetohydrodynamics code, in which a fixed mesh refinement technique is implemented. To ensure the divergence-free condition as well as the magnetic flux conservation, we employ the method by Balsara (2001). Using this new code, we evolve differentially rotating magnetized neutron stars, and find that a magnetically driven outflow is launched from the star exhibiting a kink instability. The matter ejection rate and Poynting flux are still consistent with our previous finding (Shibata et al., 2011) obtained in axisymmetric simulations.Comment: 12 pages, 14 figures, accepted by PR

Global simulations of strongly magnetized remnant massive neutron stars formed in binary neutron star mergers

We perform a general-relativistic magnetohydrodynamics simulation for $\approx 30$ ms after merger of a binary neutron star to a remnant massive neutron star (RMNS) with a high spatial resolution of the finest grid resolution $12.5$ m. First, we estimate that the Kelvin-Helmholtz instability at merger could amplify the magnetic-field energy up to $\sim 1\%$ of the thermal energy. Second, we find that the magnetorotational instability in the RMNS envelope and torus with $\rho < 10^{13}~{\rm g~cm^{-3}}$ sustains magneto-turbulent state and the effective viscous parameter in these regions is likely to converge to $\approx 0.01$--$0.02$ with respect to the grid resolution. Third, the current grid resolution is not still fine enough to sustain magneto-turbulent state in the RMNS with $\rho \ge 10^{13}~{\rm g~cm^{-3}}$.Comment: 18 pages, 10 figures, PRD in pres

Doping Dependence of Hall Coefficient and Evolution of Coherent Electronic State in the Normal State of Fe-based Superconductor Ba1−x_{1-x}Kx_{x}Fe2_{2}As2_{2}

We investigated the in-plane transport properties of the Fe-based superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ with a wide composition range $0 \leq x \leq 0.55$. We show that the doping dependence of the Hall coefficient is well-described by the Boltzmann equation for a two-band system with a rigid-band approximation. We successfully deduced transport parameters, which suggested that holes with heavier mass conduct more smoothly than electrons. Moreover, the temperature variation of the Hall coefficient indicated that an anomalous coherent state characterized by heavy quasiparticles in hole bands evolved below $\sim100$ K, predominantly in the optimal and overdoped regions. We argue that this phenomenon can be understood in relation to the pseudopeak structure observed in angle-resolved photoemission spectroscopy.Comment: 5 pages, 4 figure