Improved calculations of electron-ion Bremsstrahlung Gaunt factors for astrophysical applications

TL;DR

This paper presents precise calculations and a software tool for electron-ion Bremsstrahlung Gaunt factors, improving accuracy in astrophysical plasma modeling across non-relativistic to relativistic regimes.

Contribution

The study provides a comprehensive analysis of Gaunt factors for low-charge ions, introduces BRpack software for efficient calculations, and enhances accuracy over previous models.

Findings

Gaunt factors computed with better than 0.03% precision for Z<=2

BRpack enables efficient, accurate spectral calculations in astrophysics

Comparison shows significant improvements over earlier approximations

Abstract

Electron-ion Bremsstrahlung (free-free) emission and absorption occur in many astrophysical plasmas for a wide range of physical conditions. This classical problem has been studied multiple times, and many analytical and numerical approximations exist. However, accurate calculations of the transition from the non-relativistic to the relativistic regime remain sparse. Here we provide a comprehensive study of the free-free Gaunt factors for ions with low charge (Z<=10). We compute the Gaunt factor using the expressions for the differential cross section given by Elwert & Haug (EH) and compare to various limiting cases. We develop a new software package, BRpack, for direct numerical applications. This package uses a combination of pre-computed tables and analytical approximations to efficiently cover a wide range of electron and photon energies, providing a representation of the EH Gaunt factor to better than 0.03% precision for Z<=2. Our results are compared to those of previous studies highlighting the improvements achieved here. BRpack should be useful in computations of spectral distortions of the cosmic microwave background, radiative transfer problems during reionization or inside galaxy clusters, and the modeling of galactic free-free foregrounds. The developed computational methods can furthermore be extended to higher energies and ion charge.