General-relativistic Radiation Transport Scheme in Gmunu. I. Implementation of Two-moment-based Multifrequency Radiative Transfer and Code Tests

Abstract

We present the implementation of a two-moment-based general-relativistic multigroup radiation transport module in the General-relativistic multigrid numerical (Gmunu) code. On top of solving the general-relativistic magnetohydrodynamics and the Einstein equations with conformally flat approximations, the code solves the evolution equations of the zeroth- and first-order moments of the radiations in the Eulerian-frame. An analytic closure relation is used to obtain the higher order moments and close the system. The finite-volume discretization has been adopted for the radiation moments. The advection in spatial space and frequency-space are handled explicitly. In addition, the radiation–matter interaction terms, which are very stiff in the optically thick region, are solved implicitly. The implicit–explicit Runge–Kutta schemes are adopted for time integration. We test the implementation with a number of numerical benchmarks from frequency-integrated to frequency-dependent cases. Furthermore, we also illustrate the astrophysical applications in hot neutron star and core-collapse supernovae modelings, and compare with other neutrino transport codes.

General-relativistic Radiation Transport Scheme in Gmunu. II. Implementation of Novel Microphysical Library for Neutrino Radiation—Weakhub

“…The detailed formalism of the two-moment based radiative transfer scheme is discussed in Paper I (Cheong et al 2023). In our study, we focus on the neutrino radiation, assuming neutrinos to have zero rest mass for all flavors and ignoring neutrino oscillation effects.…”

“…While our pinpoint opacity analysis may differ from conclusions drawn using arbitrary neutrino distribution functions in fully consistent simulations, we emphasize the necessity for future studies to incorporate corrected opacities. We conducted a simulation replicating the core collapse of a 15 M e progenitor named s15s7b2, using the LS180 EOS (Lattimer & Swesty 1991), as described in Cheong et al (2023). This simulation aimed to test the implementation of conventional weak interactions in Weakhub and compare it to 3, which corresponds to the hot ring region near the core of a hypermassive neutron star remnant formed from a BNS merger system, with consideration of a tiny muon fraction.…”

“…Simulations are performed with the Harten, Lax, and van Leer Riemann solver (Harten et al 1983), second-order Minmod limiter (Roe 1986), and IMEX-SSP2(2,2,2) as the time integrator (Pareschi & Russo 2005). To minimize computational costs, the setup described in Cheong et al (2023) is adopted for the numerical treatments during different phases of a CCSN, the modes of radiation-interaction source terms treatment, the refinement setup, and the adjustment of the Courant-Friedrichs-Lewy (CFL) factor to control the time step. The computational domain extends to 10 4 km in the radial direction, with a resolution of N r = 128 and a maximum refinement level of l 12 max = (resolution in the highest refinement level is r 0.038 km max…”

“…In Paper I (Cheong et al 2023), we mentioned the formulations of the general-relativistic multifrequency radiation transport module within the General-relativistic multigrid numerical (Gmunu) code (Cheong et al 2020(Cheong et al , 2021(Cheong et al , 2022, which has been widely used in multiple applications (Ng et al 2021;Leung et al 2022;Yip et al 2023). The module is based on the two-moment general-relativistic multifrequency radiative transfer scheme (Thorne 1981;Shibata et al 2011;Cardall et al 2013a).…”

“…Here, B(ε, Ω) is the collision integral, and it differs for each type of interaction. Once the radiation four-force rad m  is obtained, we can couple it with the hydrodynamical evolution equations, accounting for energy, momentum, and lepton number exchange, following Equations (28), (29), and (116) in Cheong et al (2023). For additional information on coupling source terms to the radiation transfer module, we refer readers to Paper I (Cheong et al 2023).…”