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 (NPS) from the Global Magneto-ionic Medium Survey conducted with the Dominion Radio Astrophysical Observatory 26 m Telescope. We fit polarization angle versus wavelength squared over 2048 frequency channels from 1280 to 1750 MHz to obtain a Faraday rotation measure (RM) map of the NPS. Combining this RM map with a published Faraday depth (FD) map of the entire Galaxy in this direction, we derive the FD introduced by the NPS and the Galactic interstellar medium (ISM) in front of and behind the NPS. The FD contributed by the NPS is close to zero, indicating that the NPS is an emitting only feature. The FD caused by the ISM in front of the NPS is consistent with zero at b > 50°, implying that this part of the NPS is local at a distance of approximately several hundred parsecs. The FD contributed by the ISM behind the NPS gradually increases with Galactic latitude up to b = 44°, and decreases at higher Galactic latitudes. This implies that either the part of the NPS at b < 44° is distant or the NPS is local but there is a sign change of the large-scale magnetic field. If the NPS is local, there is then no evidence for a large-scale anti-symmetry pattern in the FD of the Milky Way. The FD introduced by the ISM behind the NPS at latitudes b > 50° can be explained by including a coherent vertical magnetic field.