Kyohei Kawaguchi

Abstract Recent detection of gravitational waves from a binary neutron star merger (GW170817) and the subsequent observations of electromagnetic counterparts provide a great opportunity to study the physics of compact binary mergers. The optical and near-infrared counterparts to GW170817 (SSS17a, also known as AT 2017gfo or DLT17ck) are found to be consistent with a kilonova/macronova scenario with red and blue components. However, in most previous studies wherein the contribution from each ejec

Abstract We perform radiative transfer simulations for kilonova in various situations, including the cases of prompt collapse to a black hole from neutron star mergers, high-velocity ejecta possibly accelerated by magnetars, and a black hole–neutron star merger. Our calculations are done employing ejecta profiles predicted by numerical-relativity simulations and a new line list for all the r -process elements. We found that: (i) the optical emission for binary neutron stars promptly collapsing t

Black hole mergers with neutron stars are promising sources of gravitational-wave signals for the new crop of gravitational wave detectors. The impressive numerical relativity simulations carried out in this paper illustrate what to expect when the orientation of the black hole spin and the equation of state of the neutron star are properly taken into account.

We develop a model for frequency-domain gravitational waveforms from inspiraling binary neutron stars. Our waveform model is calibrated by comparison with hybrid waveforms constructed from our latest high-precision numerical-relativity waveforms and the SEOBNRv2T waveforms in the frequency range of 10--1000 Hz. We show that the phase difference between our waveform model and the hybrid waveforms is always smaller than 0.1 rad for the binary tidal deformability $\stackrel{\texttildelow{}}{\mathrm

Abstract We explore the electromagnetic counterparts that will associate with binary-neutron-star mergers for the case that remnant massive neutron stars survive for ≳0.5 s after the merger. For this study, we employ the outflow profiles obtained by long-term general-relativistic neutrino-radiation magnetohydrodynamics simulations with a mean-field dynamo effect. We show that a synchrotron afterglow with high luminosity can be associated with the merger event if the magnetic fields of the remnan

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 pac

ABSTRACT We study kilonova emission from binary neutron star (BNS) mergers for the case that a remnant massive neutron star (MNS) forms and collapses to a black hole within 20 ms after the onset of the merger (which we refer to as ‘a short-lived case’) by consistently employing numerical relativity and nucleosynthesis results. We find that such kilonovae are fainter and last shorter than those for BNSs resulting in the formation of long-lived (${\gg} 1\, {\rm s}$) MNSs, in particular in the opti

We present our new general relativistic Monte Carlo (MC)-based neutrino radiation hydrodynamics code designed to solve axisymmetric systems with several improvements. The main improvements are as follows: (i) the development of an extended version of the implicit MC method for multispecies radiation fields; (ii) modeling of neutrino pair process rates based on a new numerically efficient and asymptotically correct fitting function for the kernel function; (iii) the implementation of new numerica