Kratos-polrad: Novel GPU system for Monte-Carlo simulations with consistent polarization calculations

Abstract

Polarized radiation serves as a vital diagnostic tool in astrophysics, providing unique insights into magnetic field geometries, scattering processes, and three-dimensional structures in diverse astrophysical scenarios. To address these applications, we present Kratos_polrad, a novel GPU-accelerated Monte Carlo Radiative Transfer code built upon the heterogeneous computing framework of Kratos, designed for self-consistent and efficient polarization calculations. It utilizes a comprehensive treatment of the Stokes parameters throughout photon propagation, featuring grain-lab frame transformations using quaternion algebra and a consistent analytical integration of polarized extinction and emission within each cell, which are useful in modeling radiative transfer processes with scatterings by aligned dust grains. The code implements two-step polarimetry imaging that decouples Monte Carlo sampling of scattering physics from imaging geometry, enabling efficient synthesis maximizing the utilization of photon packets. Extensive validation against analytical solutions and established codes demonstrates accurate treatment of diverse polarization phenomena, including self-scattering polarization, dichroic extinction in aligned dust grains, and complex polarization patterns in twisted magnetic field configurations. By leveraging massive GPU parallelism, optimized memory access patterns, and analytical approaches for optically thick cells, Kratos_polrad achieves performance improvements of ~10^2 times compared to CPU-based methods, enabling previously prohibitive studies in polarimetric astrophysics.

arXiv comment12 pages, 8 figures, accepted for publication in MNRAS