An alternative iterative scheme to the full non-LTE transfer problem with coherent scattering in the atom’s frame

ABSTRACT We propose an alternative scheme for the computation of the so-called ‘full non-LTE’ (non-local thermodynamic equilibrium) radiative transfer problem, assuming coherent scattering in the atom’s frame. This generalized problem should explicitly deal with the coupling between atomic velocities and the photon paths, thereby requiring the implementation of new numerical strategies to solve it. Recently, Paletou et al., presented a numerical scheme, based on the $\Lambda$-iteration, to solve this problem; however it needs to be initialized using the standard non-LTE solution with complete redistribution to achieve convergence. Our new scheme is based on accelerated $\Lambda$-iteration (ALI), which is robust and insensitive to the choice of the initial guess solution. After bench-marking our new iterative scheme against the previously developed $\Lambda$-iteration method, we demonstrate its robustness by studying its convergence behaviour. This new iterative scheme has been coded in the Julia language, and its main characteristics are hereafter described with some details.