The Rapid ASKAP Continuum Survey III: Spectra and Polarisation In Cutouts of Extragalactic Sources (SPICE-RACS) first data release

Abstract

The Australian SKA Pathfinder (ASKAP) radio telescope has carried out a survey of the entire Southern Sky at 887.5 MHz. The wide area, high angular resolution, and broad bandwidth provided by the low-band Rapid ASKAP Continuum Survey (RACS-low) allow the production of a next-generation rotation measure (RM) grid across the entire Southern Sky. Here we introduce this project as Spectral and Polarisation in Cutouts of Extragalactic sources from RACS (SPICE-RACS). In our first data release, we image 30 RACS-low fields in Stokes I, Q, U at 25 $^{\prime\prime}$ angular resolution, across 744–1032 MHz with 1 MHz spectral resolution. Using a bespoke, highly parallelised, software pipeline we are able to rapidly process wide-area spectro-polarimetric ASKAP observations. Notably, we use ‘postage stamp’ cutouts to assess the polarisation properties of 105912 radio components detected in total intensity. We find that our Stokes Q and U images have an rms noise of $\sim$ 80 $\unicode{x03BC}$ Jy PSF $^{-1}$ , and our correction for instrumental polarisation leakage allows us to characterise components with $\gtrsim$ 1% polarisation fraction over most of the field of view. We produce a broadband polarised radio component catalogue that contains 5818 RM measurements over an area of $\sim$ 1300 deg $^{2}$ with an average error in RM of $1.6^{+1.1}_{-1.0}$ rad m $^{-2}$ , and an average linear polarisation fraction $3.4^{+3.0}_{-1.6}$ %. We determine this subset of components using the conditions that the polarised signal-to-noise ratio is $>$ 8, the polarisation fraction is above our estimated polarised leakage, and the Stokes I spectrum has a reliable model. Our catalogue provides an areal density of $4\pm2$ RMs deg $^{-2}$ ; an increase of $\sim$ 4 times over the previous state-of-the-art (Taylor, Stil, Sunstrum 2009, ApJ, 702, 1230). Meaning that, having used just 3% of the RACS-low sky area, we have produced the 3rd largest RM catalogue to date. This catalogue has broad applications for studying astrophysical magnetic fields; notably revealing remarkable structure in the Galactic RM sky. We will explore this Galactic structure in a follow-up paper. We will also apply the techniques described here to produce an all-Southern-sky RM catalogue from RACS observations. Finally, we make our catalogue, spectra, images, and processing pipeline publicly available.

Prototype Faraday Rotation Measure Catalogs from the Polarisation Sky Survey of the Universe’s Magnetism (POSSUM) Pilot Observations

“…9. This footprint places the formed beams in closer overlap than the SQUARE6X6 configuration used in other ASKAP observations (Anderson et al 2021;Thomson et al 2023). The survey was observed at full resolution at the ASKAP low-band frequency range (800-1087 MHz) and averaged to a 288 MHz bandwidth with 1 MHz channel width and a central frequency of 943 MHz.…”

“…The formed beams have an approximately Gaussian response and peak sensitivity near the center, and the response becomes distorted toward the edges (Duchesne et al 2023;Thomson et al 2023). Mosaicking the beams helps achieve approximately uniform sensitivity across the majority of the image by overlapping the lower-sensitivity regions of two beams to increase the overall sensitivity to that of the central beam.…”

“…For the EM observation, field sources were used to characterize the off-axis leakage in Stokes QU (Stokes I used holography as described above). Similar methods were used by Farnsworth et al (2011) and Lenc et al (2018), and the same method that is used in this work was used by Thomson et al (2023) and is described in detail therein. In the individual formed beams of the 10 POSSUM Pilot I mid-band observations (see Section 2.1.1), the Stokes QU spectra of high signalto-noise (S/N) components (100σ in total intensity) were extracted using the method described in Section 4.1.…”

“…The amplitude of the ripple and the corresponding peaks in the Faraday spectrum increase with increasing S/N. Section 4.3 of Thomson et al (2023) provides a more detailed discussion of the amplitude of the ripple in the Faraday spectrum. While we leave a full examination of the effects of the ripple on ASKAP data for future work, we interpret any RM peaks found at these values of f with caution, particularly for high-S/N components, and suggest future users of the data presented here do the same.…”

“…The POlarized GLEAM Survey (Riseley et al 2018(Riseley et al , 2020 observed the southern sky at lower frequencies, 200-231 MHz, with a density of ∼1 RM per 80 deg 2 . Recently, the first data release from Spectra and Polarisation In Cutouts of Extragalactic sources from RACS (SPICE-RACS; Thomson et al 2023) mapped 5818 RMs across ∼1300 deg 2 of the southern sky at 744-1032 MHz. Another low-frequency survey in the northern hemisphere, the LOFAR Two-meter Sky Survey (Shimwell et al 2017;O'Sullivan et al 2023), observed from 120 to 168 MHz with an RM sky density of 0.43 RMs per square degree.…”