FARADAY TOMOGRAPHY OF THE NORTH POLAR SPUR: CONSTRAINTS ON THE DISTANCE TO THE SPUR AND ON THE MAGNETIC FIELD OF THE GALAXY

Abstract

We present radio continuum and polarization images of the North Polar Spur\n(NPS) from the Global Magneto-Ionic Medium Survey (GMIMS) conducted with the\nDominion Radio Astrophysical Observatory 26-m Telescope. We fit polarization\nangle versus wavelength squared over 2048 frequency channels from 1280 to 1750\nMHz to obtain a Faraday Rotation Measure (RM) map of the NPS. Combining this RM\nmap with a published Faraday depth map of the entire Galaxy in this direction,\nwe derive the Faraday depth introduced by the NPS and the Galactic interstellar\nmedium (ISM) in front of and behind the NPS. The Faraday depth contributed by\nthe NPS is close to zero, indicating that the NPS is an emitting only feature.\nThe Faraday depth caused by the ISM in front of the NPS is consistent with zero\nat b>50 degree, implying that this part of the NPS is local at a distance of\napproximately several hundred parsecs. The Faraday depth contributed by the ISM\nbehind the NPS gradually increases with Galactic latitude up to b=44 degree,\nand decreases at higher Galactic latitudes. This implies that either the part\nof the NPS at b<44 degree is distant or the NPS is local but there is a sign\nchange of the large-scale magnetic field. If the NPS is local, there is then no\nevidence for a large-scale anti-symmetry pattern in the Faraday depth of the\nMilky Way. The Faraday depth introduced by the ISM behind the NPS at latitudes\nb>50 degree can be explained by including a coherent vertical magnetic field.\n