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The Australian SKA Pathfinder (ASKAP) offers powerful new capabilities for studying the polarised and magnetised Universe at radio wavelengths. In this paper, we introduce the Polarisation Sky Survey of the Universe’s Magnetism (POSSUM), a groundbreaking survey with three primary objectives: (1) to create a comprehensive Faraday rotation measure (RM) grid of up to one million compact extragalactic sources across the southern 
$\sim50$% of the sky (20,630 deg
$^2$); (2) to map the intrinsic polarisation and RM properties of a wide range of discrete extragalactic and Galactic objects over the same area; and (3) to contribute interferometric data with excellent surface brightness sensitivity, which can be combined with single-dish data to study the diffuse Galactic interstellar medium. Observations for the full POSSUM survey commenced in May 2023 and are expected to conclude by mid-2028. POSSUM will achieve an RM grid density of around 30–50 RMs per square degree with a median measurement uncertainty of 
$\sim$1 rad m
$^{-2}$. The survey operates primarily over a frequency range of 800–1088 MHz, with an angular resolution of 20” and a typical RMS sensitivity in Stokes Q or U of 18 
$\mu$Jy beam
$^{-1}$. Additionally, the survey will be supplemented by similar observations covering 1296–1440 MHz over 38% of the sky. POSSUM will enable the discovery and detailed investigation of magnetised phenomena in a wide range of cosmic environments, including the intergalactic medium and cosmic web, galaxy clusters and groups, active galactic nuclei and radio galaxies, the Magellanic System and other nearby galaxies, galaxy halos and the circumgalactic medium, and the magnetic structure of the Milky Way across a very wide range of scales, as well as the interplay between these components. This paper reviews the current science case developed by the POSSUM Collaboration and provides an overview of POSSUM’s observations, data processing, outputs, and its complementarity with other radio and multi-wavelength surveys, including future work with the SKA.
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.
The radio signal transmitted by the Mars Express (MEX) spacecraft was observed regularly between the years 2013–2020 at X-band (8.42 GHz) using the European Very Long Baseline Interferometry (EVN) network and University of Tasmania’s telescopes. We present a method to describe the solar wind parameters by quantifying the effects of plasma on our radio signal. In doing so, we identify all the uncompensated effects on the radio signal and see which coronal processes drive them. From a technical standpoint, quantifying the effect of the plasma on the radio signal helps phase referencing for precision spacecraft tracking. The phase fluctuation of the signal was determined for Mars’ orbit for solar elongation angles from 0 to 180 deg. The calculated phase residuals allow determination of the phase power spectrum. The total electron content of the solar plasma along the line of sight is calculated by removing effects from mechanical and ionospheric noises. The spectral index was determined as 
$-2.43 \pm 0.11$ which is in agreement with Kolmogorov’s turbulence. The theoretical models are consistent with observations at lower solar elongations however at higher solar elongation (
$>$160 deg) we see the observed values to be higher. This can be caused when the uplink and downlink signals are positively correlated as a result of passing through identical plasma sheets.
Altered fear learning processes could be mechanistically linked to the development and/or maintenance of obsessive-compulsive disorder (OCD). From a clinical perspective, the first-line psychological treatment for OCD is cognitive-behavioral therapy (CBT), which is based on the principles of fear learning. However, no previous functional magnetic resonance imaging (fMRI) studies have evaluated the predictive capacity of regional brain activations during fear learning on CBT response in patients with OCD.
We aimed at exploring whether brain activation during fear learning in patients with OCD are associated with CBT outcome.
We assessed 18 patients with OCD and 18 healthy participants during a 2-day experimental protocol where brain activation and skin conductance responses (SCR) where assessed during fear conditioning, extinction learning, and extinction recall within the fMRI scanner. Following the protocol, patients with OCD received CBT.
We found non-significant between-group differences in SCR during fear learning. Patients with OCD showed significantly diminished activation of the dorsal anterior cingulate cortex and the right insula during fear conditioning. Importantly, our analyses revealed a significant negative association between clinical improvement after CBT and activity at the right insula during fear conditioning (x = 39, y = 12, z = -11; t = 5.64; p<0.001; k = 928). This finding is displayed in Figure 1 below.
Patients with OCD may require less fear-conditioned brain responses to achieve the same level of psychophysiological fear conditioning as healthy participants. Interestingly, insula activations during fear-conditioned responses may represent a potential predictor biomarker of response to CBT for OCD.
No significant relationships.
