Polarized Synchrotron Emissivities and Absorptivities for Relativistic Thermal, Power-Law, and Kappa Distribution Functions

TL;DR

This paper introduces a numerical scheme and fitting formulae for calculating synchrotron emissivities and absorptivities for various electron distributions, aiding astrophysical modeling of polarized emission.

Contribution

It develops a versatile numerical method and provides fitting formulae for synchrotron properties across thermal, power-law, and kappa distributions, including implementation in a C library.

Findings

Kappa distribution mimics thermal spectrum at low frequencies and transitions to self-absorbed synchrotron at high frequencies.

Thermal spectra exhibit near-unity linear polarization at high frequencies.

All distributions produce about 10% circular polarization near the magnetic field at low frequencies.

Abstract

Synchrotron emission and absorption determine the observational appearance of many astronomical systems. In this paper, we describe a numerical scheme for calculating synchrotron emissivities and absorptivities in all four Stokes parameters for arbitrary gyrotropic electron distribution functions, building on earlier work by Leung, Gammie, and Noble. We use this technique to evaluate the emissivities and the absorptivities for a thermal (Maxwell-J\"uttner), isotropic power-law, and isotropic kappa distribution function. The latter contains a power-law tail at high particle energies that smoothly merges with a thermal core at low energies, as is characteristic of observed particle spectra in collisionless plasmas. We provide fitting formulae and error bounds on the fitting formulae for use in codes that solve the radiative transfer equation. The numerical method and the fitting formulae are implemented in a compact C library called ${\tt symphony}$. We find that: the kappa distribution has a source function that is indistinguishable from a thermal spectrum at low frequencies and transitions to the characteristic self-absorbed synchrotron spectrum, $\propto \nu^{5/2}$, at high frequency; the linear polarization fraction for a thermal spectrum is near unity at high frequency; and all distributions produce $O(10\%)$ circular polarization at low frequency for lines of sight sufficiently close to the magnetic field vector.