Probing magnetic helicity with synchrotron radiation and Faraday rotation

TL;DR

This study applies the LITMUS test to detect magnetic helicity in the galactic magnetic field using synchrotron radiation and Faraday rotation, finding no helicity but highlighting the test's limitations under realistic conditions.

Contribution

It introduces the application of the LITMUS test to galactic magnetic fields and assesses its effectiveness with various electron density models and sky approximations.

Findings

LITMUS test performs well with constant electron densities.

Detection of helicity is challenging with realistic electron distributions.

No evidence of magnetic helicity was found in the galactic field.

Abstract

We present a first application of the recently proposed LITMUS test for magnetic helicity, as well as a thorough study of its applicability under different circumstances. In order to apply this test to the galactic magnetic field, the newly developed critical filter formalism is used to produce an all-sky map of the Faraday depth. The test does not detect helicity in the galactic magnetic field. To understand the significance of this finding, we made an applicability study, showing that a definite conclusion about the absence of magnetic helicity in the galactic field has not yet been reached. This study is conducted by applying the test to simulated observational data. We consider simulations in a flat sky approximation and all-sky simulations, both with assumptions of constant electron densities and realistic distributions of thermal and cosmic ray electrons. Our results suggest that the LITMUS test does indeed perform very well in cases where constant electron densities can be assumed, both in the flat-sky limit and in the galactic setting. Non-trivial distributions of thermal and cosmic ray electrons, however, may complicate the scenario to the point where helicity in the magnetic field can escape detection.