A New Multi-Energy Neutrino Radiation-Hydrodynamics Code in Full General Relativity and Its Application to Gravitational Collapse of Massive Stars

We present a new multi-dimensional radiation-hydrodynamics code for massive stellar core-collapse in full general relativity (GR). Employing an M1 analytical closure scheme, we solve spectral neutrino transport of the radiation energy and momentum based on a truncated moment formalism. Regarding neutrino opacities, we take into account a baseline set in state-of-the-art simulations, in which inelastic neutrinoelectron scattering, thermal neutrino production via pair annihilation and nucleonnucleon bremsstrahlung are included. While the Einstein field equations and the spatial advection terms in the radiation-hydrodynamics equations are evolved explicitly, the source terms due to neutrino-matter interactions and energy shift in the radiation moment equations are integrated implicitly by an iteration method. To verify our code, we first perform a series of standard radiation tests with analytical solutions that include the check of gravitational redshift and Doppler shift. A good agreement in these tests supports the reliability of the GR multi-energy neutrino transport scheme. We then conduct several test simulations of core-collapse, bounce, and shock-stall of a 15Msun star in the Cartesian coordinates and make a detailed comparison with published results. Our code performs quite well to reproduce the results of full-Boltzmann neutrino transport especially before bounce. In the postbounce phase, our code basically performs well, however, there are several differences that are most likely to come from the insufficient spatial resolution in our current 3D-GR models. For clarifying the resolution dependence and extending the code comparison in the late postbounce phase, we discuss that next-generation Exaflops-class supercomputers are at least needed.Comment: 61 pages, 20 figures, accepted for publication in ApJ

A New Gravitational-Wave Signature from Standing Accretion Shock Instabilities in Supernovae

We present results from fully relativistic three-dimensional core-collapse supernova (CCSN) simulations of a non-rotating 15 Msun star using three different nuclear equations of state (EoSs). From our simulations covering up to ~350 ms after bounce, we show that the development of the standing accretion shock instability (SASI) differs significantly depending on the stiffness of nuclear EoS. Generally, the SASI activity occurs more vigorously in models with softer EoS. By evaluating the gravitational-wave (GW) emission, we find a new GW signature on top of the previously identified one, in which the typical GW frequency increases with time due to an accumulating accretion to the proto-neutron star (PNS). The newly observed quasi-periodic signal appears in the frequency range from ~100 to 200 Hz and persists for ~150 ms before neutrino-driven convection dominates over the SASI. By analyzing the cycle frequency of the SASI sloshing and spiral modes as well as the mass accretion rate to the emission region, we show that the SASI frequency is correlated with the GW frequency. This is because the SASI-induced temporary perturbed mass accretion strike the PNS surface, leading to the quasi-periodic GW emission. Our results show that the GW signal, which could be a smoking-gun signature of the SASI, is within the detection limits of LIGO, advanced Virgo, and KAGRA for Galactic events.Comment: 7 pages, 5 figures, Accepted for publication in ApJ

Numerical Simulations of Equatorially-Asymmetric Magnetized Supernovae: Formation of Magnetars and Their Kicks

A series of numerical simulations on magnetorotational core-collapse supernovae are carried out. Dipole-like configurations which are offset northward are assumed for the initially strong magnetic fields together with rapid differential rotations. Aims of our study are to investigate effects of the offset magnetic field on magnetar kicks and on supernova dynamics. Note that we study a regime where the proto-neutron star formed after collapse has a large magnetic field strength approaching that of a ``magnetar'', a highly magnetized slowly rotating neutron star. As a result, equatorially-asymmetric explosions occur with a formation of the bipolar jets. Resultant magnetar's kick velocities are $\sim 300-1000$ km s$^{-1}$. We find that the acceleration is mainly due to the magnetic pressure while the somewhat weaker magnetic tension works toward the opposite direction, which is due to stronger magnetic field in the northern hemisphere. Noted that observations of magnetar's proper motions are very scarce, our results supply a prediction for future observations. Namely, magnetars possibly have large kick velocities, several hundred km s$^{-1}$, as ordinary neutron stars do, and in an extreme case they could have those up to 1000 km s$^{-1}$.Comment: 36 pages, 9 figures, accepted by the Astrophysical Journa

Probing mass-radius relation of protoneutron stars from gravitational-wave asteroseismology

The gravitational-wave (GW) asteroseismology is a powerful technique for extracting interior information of compact objects. In this work, we focus on spacetime modes, the so-called $w$-modes, of GWs emitted from a proto-neutron star (PNS) in the postbounce phase of core-collapse supernovae. Using results from recent three-dimensional supernova models, we study how to infer the properties of the PNS based on a quasi-normal mode analysis in the context of the GW asteroseismology. We find that the $w_1$-mode frequency multiplied by the PNS radius is expressed as a linear function with respect to the ratio of the PNS mass to the PNS radius. This relation is insensitive to the nuclear equation of state (EOS) employed in this work. Combining with another universal relation of the $f$-mode oscillations, we point out that the time dependent mass-radius relation of the PNS can be obtained by observing both the $f$- and $w_1$-mode GWs simultaneously. Our results suggest that the simultaneous detection of the two modes could provide a new probe into finite-temperature nuclear EOS that predominantly determines the PNS evolution.Comment: accepted for publication in PR