Direct Solution of the Time-Dependent Covariant Radiative Transfer Equation and its Coupling to General Relativistic Magnetohydrodynamics with cuHARM

In this paper we present a major update to the general relativistic magnetohydrodynamics (GRMHD) code cuHARM, which adds fully covariant treatment of radiation transport and the subsequent radiation backreaction on the dynamics of the fluid. For the radiative calculations, we discretize and solve the radiation transfer equation on a geodesic grid, in order to resolve the angular distribution of the radiation field everywhere in space. This allows for detailed treatment of non-isotropic radiation fields, which is crucial for accurately resolving regions of intermediate optical depth. We present the equations solved, the numerical methods used, and standard tests used to verify the different aspects of a radiation hydrodynamics code, in particular radiation transport and radiation-fluid interaction. We present an application of the code to the case of black hole radiative accretion. This new radiation module is fully GPU-accelerated and represents a major advance in the capabilities of cuHARM.