A public code for general relativistic, polarised radiative transfer around spinning black holes

TL;DR

The paper introduces grtrans, a publicly available, efficient Fortran 90 code with Python interfaces for simulating polarized radiative transfer in Kerr black hole spacetimes, aiding comparison of models with observations.

Contribution

It provides a comprehensive, verified tool for polarized radiative transfer around spinning black holes, including new fitting functions and detailed comparisons with existing literature.

Findings

The code accurately models polarized synchrotron emission.

Transfer coefficients significantly affect predicted images and polarization maps.

The code is validated through multiple tests and comparisons.

Abstract

Ray tracing radiative transfer is a powerful method for comparing theoretical models of black hole accretion flows and jets with observations. We present a public code, grtrans, for carrying out such calculations in the Kerr metric, including the full treatment of polarised radiative transfer and parallel transport along geodesics. The code is written in Fortran 90 and efficiently parallelises with OpenMP, and the full code and several components have Python interfaces. We describe several tests which are used for verifiying the code, and we compare the results for polarised thin accretion disc and semi-analytic jet problems with those from the literature as examples of its use. Along the way, we provide accurate fitting functions for polarised synchrotron emission and transfer coefficients from thermal and power law distribution functions, and compare results from numerical integration and quadrature solutions of the polarised radiative transfer equations. We also show that all transfer coefficients can play an important role in predicted images and polarisation maps of the Galactic center black hole, Sgr A*, at submillimetre wavelengths.