Probing magnetic fields with multi-frequency polarized synchrotron emission

TL;DR

This paper presents a method to reconstruct the three-dimensional structure of the Galactic magnetic field using multi-frequency polarized synchrotron emission data, leveraging internal depolarization and inverse problem techniques.

Contribution

It introduces a novel approach combining multi-band radio spectroscopy with Maximum A Posteriori reconstruction to recover large-scale magnetic field structures.

Findings

Successful recovery of magnetic field structures from simulated data.

Identification of optimal frequency sampling strategies.

Prediction of anisotropic posterior error distribution.

Abstract

We investigate the problem of probing the local spatial structure of the magnetic field of the interstellar medium using multi-frequency polarized maps of the synchrotron emission at radio wavelengths. We focus in this paper on the three-dimensional reconstruction of the largest scales of the magnetic field, relying on the internal depolarization (due to differential Faraday rotation) of the emitting medium as a function of electromagnetic frequency. We argue that multi-band spectroscopy in the radio wavelengths, developed in the context of high-redshift extragalactic HI lines, can be a very useful probe of the 3D magnetic field structure of our Galaxy when combined with a Maximum A Posteriori reconstruction technique. When starting from a fair approximation of the magnetic field, we are able to recover the true one by using a linearized version of the corresponding inverse problem. The spectral analysis of this problem allows us to specify the best sampling strategy in electromagnetic frequency and predicts a spatially anisotropic distribution of posterior errors. The reconstruction method is illustrated for reference fields extracted from realistic magneto-hydrodynamical simulations.