Science Pipelines for the Square Kilometre Array

The Square Kilometre Array (SKA) will be both the largest radio telescope ever constructed and the largest Big Data project in the known Universe. The first phase of the project will generate on the order of 5 zettabytes of data per year. A critical task for the SKA will be its ability to process data for science, which will need to be conducted by science pipelines. Together with polarization data from the LOFAR Multifrequency Snapshot Sky Survey (MSSS), we have been developing a realistic SKA-like science pipeline that can handle the large data volumes generated by LOFAR at 150 MHz. The pipeline uses task-based parallelism to image, detect sources, and perform Faraday Tomography across the entire LOFAR sky. The project thereby provides a unique opportunity to contribute to the technological development of the SKA telescope, while simultaneously enabling cutting-edge scientific results. In this paper, we provide an update on current efforts to develop a science pipeline that can enable tight constraints on the magnetised large-scale structure of the Universe.Comment: Published in Galaxies, as part of a Special Issue on The Power of Faraday Tomograph

Polarimetric observations at low radio frequencies

Magnetic fields play a fundamental role in the evolution of astrophysical systems. These fields can be studied through wide-field spectropolarimetry, which allows for faint polarised signals to be detected at relatively low radio frequencies. An interferometric polarisation mode has recently become available at the Giant Metrewave Radio Telescope (GMRT). A detailed analysis of the GMRT’s instrumental response is presented. The findings are used to create a polarisation pipeline, which in combination with rotation measure (RM) Synthesis is used for the detection of extended linearly polarised emission at 610 MHz. A number of compact sources are detected and their Faraday depth and polarisation fraction are reported for the first time. New holography observations of the GMRT’s primary beam are presented. Instantaneous off-axis polarisation is substantial and scales with the Stokes I beam. The developed beam models are used to reduce direction-dependent instrumental polarisation, and the Stokes I beam is shown to deviate from circular symmetry. A new technique for electric vector polarisation angle calibration is developed that removes the need for known sources on the sky, eliminates ionospheric effects, and avoids a flaw in current methods which could erroneously yield multiple Faraday components for sources that are well-parameterised by a single RM. A sample of nine galaxies from two Southern Compact Groups are then presented, with constraints being placed on the polarised fraction, RM, spectral index, star formation rate, companion sources, and hydrodynamical state. One galaxy has a displaced peak of radio emission that is extended beyond the disk in comparison to the near-IR disk – suggesting the radio disturbance may be a consequence of ram pressure stripping. Linear polarisation is detected from the core of NGC 7552 at 610 MHz, while another three galaxies ESO 0353–G036, NGC 7590, and NGC 7599 are found to be unpolarised. An analysis of additional extended sources allows for an FR-I and an FR-II radio source to be morphologically classified. Finally, spatial spectral variations are identified in the youngest known Galactic supernova remnant G1.9+0.3, with flatter spectra in the NW and SE. Models of cosmic ray acceleration at oblique shocks suggest the variation is most consistent with an ambient B field perpendicular to the axis of bilateral symmetry. For the first time, the presence of polarised emission is detected. There is increased ordering of the B field in the NW and strong Faraday depolarisation must also be present. An intrinsically radially-oriented field could be provided by a systematic gradient in RM of 140 rad m-2 from N to S and can also explain the depolarisation. Such a gradient may be caused by an anisotropic regular magnetic field within the remnant or in an intervening Faraday screen. The lack of strong constraints on the RM, and the remnant’s current evolutionary stage, leave open the possibility that Rayleigh–Taylor instability formation has not yet fully taken place

The extraordinary linear polarisation structure of the southern Centaurus A lobe revealed by ASKAP

We present observations of linear polarisation in the southern radio lobe of Centaurus A, conducted during commissioning of the Australian Square Kilometre Array Pathfinder (ASKAP) telescope. We used 16 antennas to observe a 30 square degree region in a single 12 hour pointing over a 240 MHz band centred on 913 MHz. Our observations achieve an angular resolution of $26\times33$ arcseconds (480 parsecs), a maximum recoverable angular scale of 30 arcminutes, and a full-band sensitivity of 85 \muupJy beam$^{-1}$. The resulting maps of polarisation and Faraday rotation are amongst the most detailed ever made for radio lobes, with of order 10$^5$ resolution elements covering the source. We describe several as-yet unreported observational features of the lobe, including its detailed peak Faraday depth structure, and intricate networks of depolarised filaments. These results demonstrate the exciting capabilities of ASKAP for widefield radio polarimetry.Comment: 10 pages, 6 figures. Accepted in "The Power of Faraday Tomography" special issue of Galaxie

Faraday Tomography of the SS433 Jet Termination Region

A jet termination region provides us with useful information about how a jet interacts with the interstellar medium. Identifying the strength and orientation of magnetic fields at the terminal is crucially important to understanding the mechanism of cosmic-ray acceleration. In this article, we report results of our Faraday-tomography analysis of the eastern region of the radio nebula W50, where a jet from the microquasar SS433 seems to terminate. We apply QU-fitting, a method of Faraday-tomography, to data from the Australia Telescope Compact Array (ATCA) at 1.3–3.0 GHz. In the analysis, we distinguish multiple polarized sources along the line of sight. We identify Galactic emission candidates at Faraday depths around 0 rad m−2 and 300 rad m−2. The Galactic emission around 0 rad m−2 is possibly located in front of W50. We also find emission from W50 with Faraday depths between φ = −112 to 228 rad m−2