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POSSUM: Polarisation Sky Survey

of the Universe's Magnetism



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About POSSUM:

The Australian Square Kilometre Array Pathfinder (ASKAP) is a next-generation radio telescope about to commence operations in outback Australia. ASKAP's large instantaneous field-of-view allows it to survey the sky vastly faster than is possible with existing radio telescopes, allowing us to address fundamental questions about the origin and evolution of the Universe.

The "Polarisation Sky Survey of the Universe's Magnetism" (POSSUM) is one of the eight major surveys being conducted on ASKAP. It is a continuum polarisation survey of the entire southern sky, primarily covering ASKAP's "Band 1" frequency range (800–1088 MHz), conducted commensally with the Evolutionary Map of the Universe survey (EMU). Supplementary observations over one-third of the sky will also be carried out in ASKAP's "Band 2" frequency range (1130–1430 MHz), commensal with the Widefield ASKAP L-Band Legacy All-Sky Blind Survey (WALLABY). The survey will reach an RMS sensitivity of 17 μJy/beam at 20-arcsecond resolution (in Band 1). POSSUM will deliver:

  • A catalogue of Faraday rotation measures (RMs) for approximately one million extragalactic radio sources, at an average density of ~30 RMs per deg<sup>2</sup>;
  • Full-Stokes spectral measurements for all sources within the survey area;
  • Spatially resolved polarisation maps — including 3D Faraday synthesis products — across the full survey footprint.

These data will enable us to:

  • Map the 3D geometry of the Milky Way’s magnetic field;
  • Test models of magnetic field generation, including dynamos and primordial seeding;
  • Perform a statistical census of magnetic fields in galaxies, clusters, and the intergalactic medium as a function of redshift;
  • Explore the magnetised properties of a wide range of Galactic and extragalactic sources.

The survey is described in the Gaensler et al. 2025 Survey Description Paper (arXiv; in press at PASA)

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