GMIMS-DRAGONS: A Faraday Depth Survey of the Northern Sky Covering 350 to 1030 MHz

TL;DR

The DRAGONS survey advances Galactic magnetism studies by delivering a broadband (350–1030 MHz) Faraday-depth dataset for the Northern sky with the DRAO-15 m telescope. It combines wideband polarization measurements, careful RFI mitigation, and ionospheric RM corrections to produce calibrated Stokes I, Q, and U cubes and derived Faraday-depth cubes (with RM synthesis and RM-CLEAN) and Faraday-moment maps. The work demonstrates substantial Faraday complexity across ~55% of sightlines and validates the data against legacy surveys, enabling robust cross-comparisons and multi-resolution synergy with CHIME and LoTSS data. DRAGONS provides a critical, high-signal resource for constraining the 3D Galactic magnetic field, ground-emission models, and future wideband polarization studies, with future improvements planned in ground modeling and basket-weaving, and strong potential for integration with higher-resolution surveys.

Abstract

Polarized synchrotron emission at meter to centimeter wavelengths provides an effective tracer of the Galactic magnetic field. Calculating Faraday depth, the most useful parameter for mapping the line-of-sight magnetic field, requires observations covering wide frequency bands with many channels. As part of the Global Magneto-Ionic Medium Survey we have observed polarized emission spanning 350-1030 MHz over the Northern sky, in the declination range ${-20^{\circ}}\leqδ\leq{90^{\circ}}$. We used the 15 m telescope at the Dominion Radio Astrophysical Observatory (DRAO), equipped to receive orthogonal circular polarizations, with the Onsala Space Observatory band-1 feed developed for the Square Kilometre Array. Angular resolution varies across the band from $1.3^{\circ}$ to $3.6^{\circ}$. A digital spectrometer provided 42 kHz frequency resolution. Data were taken with the telescope moving rapidly in azimuth and are absolutely calibrated in intensity. Approximately 20% of the band was obscured by radio-frequency interference. Resolution in Faraday depth is $\sim6$ rad m$^{-2}$, and features as wide as $\sim38$ rad m$^{-2}$ are represented. The median sensitivity of the Faraday depth cube is 11 mK. Approximately 55% of sight-lines in this survey show Faraday complexity. This dataset, called ``DRAO GMIMS of the Northern Sky'' (DRAGONS), is the first to probe Faraday depth of the Northern sky in its frequency range and will support many scientific investigations. The data will be used to calibrate surveys with higher angular resolution, particularly Galactic foreground maps from the Canadian Hydrogen Intensity Mapping Experiment, and to provide information on large structures for aperture-synthesis telescopes, particularly the DRAO Synthesis Telescope. The data are available through the Canadian Astronomy Data Centre.

Figures (38)

• Figure 1: The DRAO-15 telescope, a ${15}\times{18}$ m offset Gregorian reflector (top) and the Onsala Space Observatory quad-ridged, flared horn feed (bottom).
• Figure 2: The measured HPBW of the telescope as a function of frequency, determined from the FWHM of Gaussians fitted to raster scans of compact sources (see Section \ref{['subsubsec:raster']}). Gaps in the measured beamwidth data are channels with too much RFI for a successful Gaussian fit. A boxcar median smoothing kernel of width 49 channels ($\sim4$ MHz) was applied to the measured FWHM values.
• Figure 3: Frequency dependence of pointing measurements using the average of seven Cyg A raster scans. (a) Right ascension offset, (b) declination offset, (c) difference in right ascension measured in LL* and RR*, and (d) difference in declination measured in LL* and RR*. The consistently positive offsets in (a) and (b) are the residual, systematic errors in the pointing accuracy following the correction described in Section \ref{['subsec:pointing']}. The 'persistent RFI' mask described in Section \ref{['subsec:rfi']} was applied to these data to eliminate most RFI channels. Remaining outliers are due to poor-quality fits to the raster scans in channels contaminated by intermittent RFI.
• Figure 4: Right ascension (top) and declination (bottom) offsets between the expected and observed pointing positions as a function of azimuth. These measurements are derived from Gaussian fits to the calibration raster scans of Cyg A, Cas A, Tau A, and Vir A (Section \ref{['subsubsec:raster']}) produced over the course of the survey.
• Figure 5: The receiver noise temperature in kelvins (top) and the kelvin equivalent of the calibration noise source (bottom) as determined from the HCTF measurements. Results are shown for both right- and left-hand circular polarization (RCP; black and LCP; grey).