Chorus: optimizing synchrotron transfer coefficients with weighted sums

ABSTRACT Accurate synchrotron transfer coefficients are essential for modelling radiation processes in astrophysics. However, their current calculation methods face significant challenges. Analytical approximations of the synchrotron emissivity, absorptivity, and rotativity are limited to a few simple electron distribution functions that inadequately capture the complexity of cosmic plasmas. Numerical integrations of the transfer coefficients, on the other hand, are accurate but computationally prohibitive for large-scale simulations. In this paper, we present a new numerical method, Chorus, which evaluates the transfer coefficients by expressing any electron distribution function as a weighted sum of functions with known analytical formulas. Specifically, the Maxwell–Jüttner distribution function is employed as the basic component in the weighted sum. The Chorus leverages the additivity of transfer coefficients, drawing inspiration from an analogous approach that uses stochastic averaging to approximate the $\kappa$ distribution function. The key findings demonstrate median errors below $5{{\ \rm per\ cent}}$ for emissivity and absorptivity, with run times reduced from hours to milliseconds compared to first-principles numerical integrations. Validation against a single $\kappa$ distribution, as well as its extension to more complicated distributions, confirms the robustness and versatility of the method. However, limitations are found, including increased errors at higher energies due to numerical precision constraints and challenges with rotativity calculations arising from fit function inaccuracies. Addressing these issues could further enhance the method’s reliability. Our method has the potential to provide a powerful tool for radiative transfer simulations, where synchrotron emission is the main radiative process.