Spectra and Polarisation in Cutouts of Extragalactic sources from RACS first data release

NOTICE June 2024: We have discovered an error in the flux density of Stokes Q and U in this data release. Full details will be provided in a corrigendum in PASA. If you are reading this notice the catalogue, spectra and images in the CASDA collection have now been corrected. Note that this link will take you to ‘v4’ of the collection, published in June 2024, which has the corrected data. Collections published before this date, or with version less than ‘v4’, do not have the correction applied. The data in the linked ‘full’ DAP collection still need to be corrected. We will update this notice once the corrected data is uploaded there. Users of the old data can apply their own correction by multiplying Stokes Q and U by 2 for correct results. However, we recommend using the updated products provided in the CASDA deposit.

The first linear polarisation products from RACS are now available from the first data release (DR1) of SPICE-RACS. We derived this first release from a subset of 30 fields from RACS-low1, which covers approximately 1300 square degrees contiguously. The full set of a data products are available on CASDA and the CSIRO Data Access Portal (DAP).

Unlike previous RACS releases, SPICE-RACS does not provide images over the entire ASKAP field of view. Rather, we produce small cutouts ‘cubelets’ around each source identified in the total intensity catalogue. In SPICE-RACS DR1, we have produced image cublelets with 1MHz spectral resolution, convolved to a common resolution of 25 arcsec, in Stokes I, Q, and U. We have performed a correction for both wide-field leakage and ionospheric Faraday rotation in the image plane. On the total intensity peak of each RACS-low1 component we extract the polarisation spectra, perform rotation measure (RM) synthesis, and save the resulting spectra in the PolSpectra format. Finally, we have compiled a polarisation catalogue in the RMTable standard.

On the left is the tiling on the RACS-low1 sky. Centred on the tiling are 30 fields, highlighted with a darker colour. On the right is an inset zoom, showing the distribution of the SPICE-RACS DR1 RMs with the tiling grid overlaid.

Available data products

Due to the large size of the SPICE-RACS DR1 data, we have split our data release into two subsets. For each subset, the polarisation catalogues contains a row for all total intensity component. It is critical to note that the polarisation information will not be reliable for all rows in our catalogues. Please see the ‘Recommended catalogue subsets’ section below for a guide on how to use our catalogues appropriately for science.

First, for the brightest 24680 total intensity components we provide full CASDA collection, which is available at this permalink. The catalogues in this set have the ‘cut‘ tag in the title. This collection contains:

  • {source_id}.cutout.{image,weights}.*.fits: Image and weights cube cutout in Stokes I, Q, and U cutout around a RACS-low source identified by ‘source_id‘. The data dimensions are J2000 right ascension and declination, Stokes parameter, and frequency.
  • {cat_id}_polspec.fits: Polarisation spectra extracted on the position of the RACS-low component identified by ‘cat_id‘ in POLSPECTRA format. Each table contains a single row, corresponding to the component.
  • spice-racs.dr1.corrected.cut.xml: RMTable polarisation catalogue in VOTable format
  • RACS_DR1_Sources_spice-racs.dr1.corrected.cut.xml: A subset of the RACS-low1 DR1 source catalogue corresponding to the position information of each cutout cubelet.

In addition to this collection, we provide the superset of products derived from all 105912 total intensity components. Whilst the data products are fundamentally same as above, they are collected slightly differently. The catalogues have the ‘full’ tag in the title, all spectra are collected in a monolithic tarball, and cubelets are collected in tarballs split approximately by declination. These data products are available within a CSIRO DAP collection:

Recommended catalogue subsets

Our ‘full’ catalogue contains extracted polarisation data for all 105912 Stokes I components detected in our DR1 region. It is important to note that the majority of these do not have reliably detected polarised emission. When using this catalogue please use the provided flags, and uncertainty/quality metrics to derive a subset suitable for your science goals. If you’re unsure where to start, these are our recommended basic subsets:

  • goodI‘ : Where our Stokes I model is reliable — channel_flag and stokesI_fit_flag are False (this produces the ‘cut‘ catalogue from the ‘full‘).
  • goodL‘: Where we have a reliable detection of polarised signal, but not necessarily a reliable RM – goodI plus leakage_flag is False and snr_polint >= 5.
  • goodRM‘: Where the RM is reliable – goodL plus snr_flag is False.