Detecting Helical Magnetic Fields with Modern Radio Telescopes

by Axel Brandenburg 31 Oct 2014 Science News
Published in the "From the Institute" section of Nordita News issue 1, 2014

For many decades, polarized radio emission from external galaxies has been used to infer the strength and structure of their magnetic field. This emission is caused by relativistic electrons gyrating around magnetic field lines and producing the polarized synchrotron emission. The plane of polarization gives an indication about the electric (and thus magnetic) field vectors at the source of emission. The line-of-sight component of the field can be inferred through the Faraday effect that leads to a wavelength-dependent rotation of the plane of polarization.

Set of cells each with a singly helical magnetic field of positive helicity. (From arXiv:1401.4102)

In practice, an observer will always see a superposition of different polarization planes from different depths, which can lead to a reduction in the degree of polarization. Firstly, the orientation of the magnetic field changes, causing different polarization planes at different positions. Secondly, Faraday rotation causes the plane of polarization to rotate. The decrease in polarized emission resulting from this superposition is referred to as Faraday depolarization. This was regarded as a problem that can be alleviated partially by restricting oneself to observations at shorter wavelengths. This situation has changed with the advent of new generations of radio telescopes that can measure polarized emission over a broad and continuous range of wavelengths. This allows one to apply the method of Burn (1966, Mon. Not. Roy. Astron. Soc. 133, 67) that utilizes the wavelength-dependent depolarization to determine the distribution of radio sources with respect to Faraday depth.

This was also the topic of a one-week workshop organized by form Nordita fellow Oliver Gressel in September 2013. As a direct outcome of this workshop, two papers have now appeared that examine the possibility of detecting helical magnetic fields in the radio sky; one is by the Nordita astrophysics research committee member Cathy Horellou (Onsala) together with Andrew Fletcher (Newcastle) and the other one is by Axel Brandenburg (Nordita) and Rodion Stepanov (Perm). Both papers appeared on 16 January on the arXiv (arXiv:1401.4152 and arXiv:1401.4102) and have now been published.