The Polarization Sky Survey of the Universe’s Magnetism (POSSUM) is a project that will use the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope, located on CSIRO’s Murchison Radio-astronomy Observatory (MRO) in remote Western Australia. POSSUM will study magnetic fields in various environments across the Universe, through synchrotron radiation and its associated Faraday rotation. The novel capabilities of ASKAP will allow us to form a magnetic picture of the Universe with unprecedented detail. A key concept is the measurement of Faraday rotation measures (RMs), a property of radio sources that tells us the strength and direction of magnetic fields along the line of sight.

Faraday rotation is imprinted on radio signals from distant sources by intervening regions along the line of sight with magnetic fields and ionized gas. The sign of the Faraday rotation measure is positive if the magnetic field points toward us, or negative if the field points away. POSSUM can measure even weak magnetic fields and very low-density ionized gas. Graphic courtesy of Emma Alexander.

The POSSUM survey requires exquisite full-polarization performance from ASKAP’s novel widefield Phased Array Feed (PAF) system. Our team is directly involved in characterising and correcting instrumental errors that can mask the astrophysical signals of interest. POSSUM members are also instrumental in developing new, efficient algorithms for extracting detailed polarization characteristics from the large, broad bandwidth data cubes generated by the ASKAP pipeline.

These techniques are being developed and validated through a progression of observational stages that will lead to the full POSSUM survey.

POSSUM Pilot Surveys

There are two POSSUM Pilot Surveys, each comprising about 100 hours of observing time.

POSSUM Pilot Phase 1

Our first pilot survey was chosen to overlap the sky coverage of the first EMU pilot survey, itself a contiguous patch of about 270 square degrees within the Dark Energy Survey area, with a complementary frequency range. EMU covered 800-1088 MHz, while POSSUM covered 1224-1502 MHz. The sky area covered by POSSUM was slightly smaller than the EMU footprint due to the smaller ASKAP field of view at higher frequency.

The sky coverage of POSSUM pilot phase 1, overlaid on the CHIPASS continuum map in Galactic coordinates and on an Aitoff projection.

POSSUM Pilot Phase 2

The sky coverage and frequency bands for the second POSSUM pilot survey were selected to test various aspects of detailed planning for the full POSSUM survey, and specifically to answer some outstanding questions around commensality with the EMU and WALLABY surveys.

Here, “Band 1” refers to 800-1088 MHz, and “Band 2” refers to 1152-1440 MHz.

  • Shapley supercluster: two tiles, 10h each in both Bands 1 and 2
  • ACT fields: four tiles, 10h each in Band 1
  • Small Magellanic Cloud: 1 tile, 10h in both Bands 1 and 2
  • Galactic supernova field: 1 tile, 10h in both Bands 1 and 2
  • EMU+POSSUM+WALLABY commensality test field: 1 tile, 10h in Band 1 and 2x8h (interleaved) in Band 2


The Rapid ASKAP Continuum Survey (RACS) has imaged the entire Southern radio sky from 744-1032 MHz, and will be extended to also cover 1296-1440 and ~1600-1800 MHz. A total intensity catalog produced from the low-frequency RACS observations (published here) contains entries for about 2.1 million sources, making it the largest radio continuum survey in the Southern hemisphere to date. The POSSUM team is delivering linear polarization products from the RACS data, through the SPICE-RACS (Spectra and Polarization In Cutouts of Extragalactic Sources from RACS) project (track developments here).

When complete, we expect to recover approximately 2-5 RMs per square degree over the full 30,000 square degree RACS survey area, which is about a factor of 10 lower source density than is expected from the full POSSUM survey, but still 2-5x denser than the state-of-the-art NVSS RM catalog.

POSSUM all-sky survey

The exact definition of the full POSSUM survey awaits high-level decisions to define the frequency coverage and sky area of the two other all-sky ASKAP surveys, EMU and WALLABY. POSSUM will be commensal with both of these projects, provided that computational resources are sufficient, as projected. Our ideal design outcome is an all-sky survey in both Band 1 (commensal with EMU) and Band 2 (commensal with WALLABY). Final details will be provided here upon the release of outcomes from the ongoing Review of ASKAP Survey Science Projects (RASSP) process.