Limits to the magnetic field in the planetary nebula NGC 246 from faraday rotation

Artikel i vetenskaplig tidskrift, 2017

We present radio continuum observations of the linearly polarized extragalactic source J0047-1150, whose line of sight traverses the galactic planetary nebula NGC 246. We determine the position angle of the electric vector at seven frequencies between 1.3 and 1.8 GHz, finding no evidence of Faraday rotation and setting a 4-sigma upper limit to the rotation measure of 9.6 rad m(-2), which implies an upper limit to the average line-of-sight component of the magnetic field in NGC 246 of 1.3 mu G. However, we show that the rotation measure across a source with a dipolar magnetic field morphology practically cancels out. Therefore, if the magnetic field has this morphology, the local values of the magnetic field in NGC 246 could be much larger and will not be evident in a Faraday rotation experiment.