Toward First-principles Multimessenger Predictions: Coupling Nuclear Networks with General Relativistic Radiation Magnetohydrodynamics in <tt>Gmunu</tt>

Abstract We present a new implementation of nuclear reaction networks in the general relativistic mu ltigrid nu merical ( Gmunu ) code, a framework for general relativistic radiation magnetohydrodynamics (GRRMHD). The extended code self-consistently evolves nuclear species fully coupled to hydrodynamics, magnetic fields, and neutrino radiation transport under the conformal flatness approximation to Einstein’s equations. Four approximate nuclear networks are incorporated, with stiff source terms integrated implicitly using implicit–explicit Runge–Kutta schemes. Validation is performed through a suite of benchmarks, including conserved-to-primitive recovery with a tabulated stellar equation of state, one-zone silicon burning, and hydrodynamic tests of shock tubes, acoustic pulses, and detonation fronts of Type Ia supernovae. These tests confirm accurate coupling between nuclear reactions and fluid dynamics, conserving both electron and nuclear mass fractions to machine precision. As an application, we perform spherically symmetric core-collapse supernova simulations. The models reproduce the expected nonexploding behavior of standard progenitors, while enhanced neutrino heating leads to shock revival. Including nuclear burning further alters the postshock composition and dynamics, converting silicon and oxygen layers into iron-group nuclei and strengthening the explosion. This demonstrates the impact of explosive burning on both ejecta composition and shock evolution, and establishes the stability of the coupled GR radiation–MHD–nuclear framework. Although magnetic fields are not evolved in the present 1D application, the implementation is fully compatible with multidimensional GRRMHD simulations. This work represents the first GRRMHD code combining M1 neutrino transport with fully coupled nuclear burning, paving the way for multidimensional studies of supernovae and compact object mergers where nucleosynthesis feedback shapes multimessenger signals.