The Global Magneto-ionic Medium Survey: A Faraday Depth Survey of the Northern Sky Covering 1280–1750 MHz

Abstract

The Galactic interstellar medium hosts a significant magnetic field, which can be probed through the synchrotron emission produced from its interaction with relativistic electrons. Linearly polarized synchrotron emission is generated throughout the Galaxy and, at longer wavelengths, modified along nearly every path by Faraday rotation in the intervening magneto-ionic medium. Full characterization of the polarized emission requires wideband observations with many frequency channels. We have surveyed polarized radio emission from the Northern sky over the range 1280–1750 MHz, with channel width 236.8 kHz, using the John A. Galt Telescope (diameter 25.6 m) at the Dominion Radio Astrophysical Observatory, as part of the Global Magneto-Ionic Medium Survey. The survey covered 72% of the sky, decl. −30° to +87° at all R.A. The intensity scale was absolutely calibrated, based on the flux density and spectral index of Cygnus A. Polarization angle was calibrated using the extended polarized emission of the Fan Region. Data are presented as brightness temperatures with angular resolution 40′. Sensitivity in Stokes Q and U is 45 mK rms in a 1.18 MHz band. We have applied rotation measure synthesis to the data to obtain a Faraday depth cube of resolution 150 rad m−2 and sensitivity 3 mK rms of polarized intensity. Features in Faraday depth up to a width of 110 rad m−2 are represented. The maximum detectable Faraday depth is ±2 × 104 rad m−2. The survey data are available at the Canadian Astronomy Data Centre.

A Unified Model for the Fan Region and the North Polar Spur: A bundle of filaments in the Local Galaxy

“…As discussed in the previous section, we assume a magnetic field strength of 24 µG, but note that the bulk of this field is in the plane-of-sky, rather than along the line-of-sight. These values give FDs of a few rad m 2 , which is consistent with observations (Spoelstra 1984;Sun et al 2015;Wolleben et al 2021).…”

“…On the other hand, for nearby emission, the path length is short, then the amount of Faraday rotation should be smaller, and we would expect greater agreement between the two maps. This is another way of showing that the Faraday rotation towards these bright polarized regions is small, which has been shown by other authors (Spoelstra 1984;Sun et al 2015;Wolleben et al 2021). Small FD in the direction of the Fan Region is not unexpected since, from both modelling and observations, we expect that the Galactic magnetic field in this direction is dominated by the plane-of-sky magnetic field (B ⊥ ), whereas the line-of-sight field (B ) is small.…”

“…The enhancement relative to total intensity is largely due to the field orientation perpendicular to the line of sight. The small FD values of a few rad/m 2 observed by Spoelstra (1984) and also Sun et al (2015); Wolleben et al (2021) suggest that it is nearby.…”

An Overview of CHIME, the Canadian Hydrogen Intensity Mapping Experiment

“…Exploration of this parameter space is only beginning (Dickey et al 2019;Thomson et al 2019). The 400-800 MHz polarization maps with ≈ 40 angular resolution from CHIME will form a component of the GMIMS survey, which includes a southern sky dataset obtained with the CSIRO Parkes Telescope (Wolleben et al 2019) and a 1280 to 1750 MHz northern sky dataset observed with the Galt Telescope (Wolleben et al 2021). If we are able to combine data across the 400 to 1800 MHz range, we will achieve δφ ≈ 7 rad m −2 and φ max−scale ≈ 110 rad m −2 , providing sensitivity to an unprecedented range of Faraday depth scales.…”

“…This dataset will be without precedent in the Northern hemisphere and will form a component of the Global Magneto-Ionic Medium Survey (GMIMS). GMIMS is the first effort to measure the all-sky three-dimensional structure of the Galactic magnetic field, using telescopes around the world to obtain maps with sensitivity to the range of Faraday depth structures we expect in the diffuse medium (Wolleben et al 2019(Wolleben et al , 2021; the CHIME frequency range is a critical component of GMIMS.…”