A numerically stable comoving frame solver for line radiative transfer

Radiative transfer is essential in astronomy, both for interpreting observations and simulating various astrophysical phenomena. However, self-consistent line radiative transfer is computationally expensive, especially in 3D. To reduce the computational cost when utilizing a discrete angular discretization, we use a comoving frame interpretation of the radiative transfer equation. The main innovation of this paper lies in the novel stabilization method for the resulting numerical discretization. The stabilization method is able to reduce spurious oscillatory behavior in the computed intensities, at the expense of extra boundary conditions which need to be enforced. We also implement an adaptive angular discretization for the ray-tracing implementation, in order to efficiently and accurately calculate the radiation field. Finally, we apply this new numerical method to compute NLTE line radiative transfer on a hydrodynamics model, showcasing its potential improvement in computation efficiency.