MWA Report

MWA Phase II Upgrade

Over the last two years, thousands of new antennas have been added to the Murchison Widefield Array (MWA) in an upgrade known as Phase II. This gave the array the flexibility to choose between different modes, ‘compact’ or ‘extended’, based on which sets of antennas are used to observe with. In August, the MWA concluded its first observing campaigns in the ‘extended’ configuration. With baseline distances up to 6km, the telescope’s sensitivity in this mode improved by a factor of 10 (compared to Phase I).

The motivation and science details of the upgrade are described in The Phase II Murchison Widefield Array: Design Overview by Wayth et al, 2018. The antennas in the compact configuration enhance the surface brightness sensitivity of the MWA and will improve the ability of the MWA to estimate the slope of the Epoch of Reionisation power spectrum by a factor of ~3.5. The long baseline tiles improve the array u,v coverage, giving an order of magnitude improvement in the noise floor of MWA continuum images, resulting in more resolved images such as the one shown below.

Figure 1: 185 MHz images of Fornax-A taken with the Phase I (left) and Phase II (right) MWA. (2018, Wayth et al.)

With this upgrade, the MWA will be able to improve detections of missing supernova remnants, push detection of diffuse emission (halos and relics) in galaxy clusters back to a redshift of 1 (Phase 1 was limited to objects with z < 0.45), explore the life cycles of active galactic nuclei (AGN) in more detail, particularly via the detection of so-called ‘dead’ or ‘restarted’ radio galaxies, and potentially detect the diffuse radio emission of the cosmic web.

On the technical side, improving the resolution and sensitivity of the instrument requires improvements to the MWA processing pipelines. This is to incorporate direction dependant effects, build better foreground and ionospheric models (which are useful as science products in their own right) and develop scalable processing regimes. All of these things will also be of direct relevance to SKA-low..

Furthermore, there is ongoing work to i) design a new correlator capable of ingesting all 256 tiles, ii) improve the current receiver systems, iii) build better monitoring and control software, and iv) further refine the operation of the MWA archive and data storage & retrieval platform as part of the Australian All-Sky Virtual Observatory (ASVO). Undertaking this work is of direct relevance to the SKA, and the MWA provides a prototyping platform for much of the SKA pre-construction work, as well as a training-ground for developing relevant skills to design, construct and exploit SKA-low.

The last quarter included the first telescope reconfiguration between functioning states (extended to compact), and the array downtime during the commissioning period allowed for upgrades to the telescope back-end, such and the monitor and control system. The contractor-led maintenance program played a pivotal role in ensuring high levels of telescope availability and performance throughout the observing semester.

Off-site work has continued with the development of new software and hardware for Phase III of the telescope, with test observations using the new correlator identifying individual pulsar pulses. Substantive progress has also been made with the new receiver candidate, which is now undergoing field-tests and final firmware revisions. Additionally, new industry collaborations have been forged to locally manufacture hardware and CAD drawings, and online visibility and access to the MWA has been significantly improved with the development of a new website, wiki, and collaboration membership system.

MWA Node of the All-Sky Virtual Observatory

Over the last year we have developed the MWA node of the All-Sky Virtual Observatory (ASVO), an Australian project to provide data portals for significant Australian telescopes. The MWA node allows the MWA Collaboration and others to access 28 Petabytes of stored MWA data. Since its inception in November 2017, the MWA ASVO pilot has served 639 TB of data and completed over 27,000 jobs.

The pilot has been highly successful and will soon be adopted as the official MWA data retrieval method, replacing previous internal processes. At present the service only provides raw, uncalibrated visibilities; future development of the MWA ASVO includes the possibility to serve calibrated visibilities as well as potentially an imaging pipeline.

MWA at the MRO Open Day

The MWA team was present on site at the recent Murchison Radio-astronomy Observatory (MRO) Open Days, the first public showing of the Australian SKA precursor telescopes. Director Melanie Johnston-Hollitt, Project Officer Mia Walker and Data Manager Greg Sleap gave tours of the MWA array core, correlator room, and additional SKA frontrunner projects AAVS and EDA.

Figure 2: MWA and CSIRO personnel at the MRO Open Day, October 2018. Image credit: CSIRO

MWA Science

The Phase I MWA produced over 110 refereed publications in its 5 years of operations (2012 – 2017), with a further 50 papers published since Phase II commenced in late 2017, taking the total number of MWA publications to over 160. To date these publications have accrued nearly 5000 citations (Figure 3), at a growing rate of citations per year with ~1300 citations being generated in the first 10 months of 2018.

Figure 3: MWA Publications

Recent highlights include:

LEAP: An innovative direction-dependent ionosphere calibration methodfor low frequency arrays

The ambitious scientific goals of the SKA require a matching capability forcalibrationof the systematic errors that contaminate the observed signals. M. Rioja and R. Dodson recently visited the SKA H/Q and presented a scheme, LEAP, for addressing the direction-dependent (DD) ionospheric and instrumental phase effects at the low frequencies and with fields of view planned for SKA-Low (Rioja et al. 2018, MNRAS, 478, 2337).

LEAP is a perfectly parallel process; that is,multiple directions can be processed independently and simultaneously, as it does not depend on a complete sky model. Using MWA Phase I observations at 88 and 154 MHz under various weather conditions, LEAP has been shown to be able to measure and correct for temporal and direction-dependent spatial distortions on a wide range of scales, including those comparable to or less than the array size.

Initial analysis of MWA Phase IIobservations, with baselines up to 6 km, shows that the incidence of higher order spatial ionospheric distortions becomeeven more significant on short timescales or on scales comparable to or smaller than the array size. Such ionospheric distortions can only be corrected for in the visibility domain, so a DD-correction scheme such as LEAP is imperative for MWA Phase II. Furthermore, LEAP it is expected to scale well (being perfectly parallel) to the requirements of SKA-Low.

Shown are the LEAP ionospheric phase surfaces from severely ionospherically-contaminated 2-minute snapshot data from the extended array, in June 2018 at 150 MHz. The solution interval is 30s and show the scale of variations and their rapid evolution.

Figure 4. Comparison of the image quality: The distortions were sufficient to introduce PSF residuals from strong sources across the whole field of view. Shown are two images, both scaled from -0.1 to 1 Jy, around the ca. 40 Jy source 3C444. A single contour level at 0.1 Jy is overlaid. On the left is the output of GLEAM-X standard analysis, which was rated as unusable; inset is a close-up around 3C444, showing more detail of the residuals. On the right, after applying the LEAP corrections, the image quality is sufficiently recovered to be useable. Also, the LEAP calibration results in a 28% peak flux increase in the image.

Figure 5, a-d: Sequence of four consecutive, 30-second long, Direction Dependent (DD) Ionospheric screens, measured with LEAP above the MWA-Phase 2 array, for a given direction within the FoV at 150 MHz. The Z-axis represents the DD ionospheric phase errors (in degrees) and the X,Y-axis are the antenna coordinates in metres. They suggest a change in character compared to those from MWA Phase 1 observations with shorter baselines: significant higher order ionospheric phase structure is seen over the array, on a finer scale than the size of the array, along with significant temporal fluctuations. In these cases the dominant image artefact is defocusing, in addition to the well-known position shifts, and calibration in the visibility domain is required.

Figure 6, e-f: Images of the same source before (left) and after (right) DD LEAP calibration using the ionospheric screens shown to correct for the DD distortions. The LEAP calibration results in a 25% peak flux increase in the image.

The Murchison Widefield Array (MWA) is a low-frequency radio telescope in Western Australia, and a precursor instrument to the Square Kilometer Array (SKA). It consists of thousands of spider-like antennas arranged in regular grids called ‘tiles’, spread over several kilometers within the Murchison radio-astronomy observatory. The MWA is renowned for its wide field of view and nanosecond time resolution, making it invaluable for quickly mapping the sky and studying rare and faint events as they happen. The MWA is run by a consortium of 21 institutions from Australia, New Zealand, China, Japan, Canada and the USA, led by Curtin University. Funding is provided by partner institutions and the Australian Government under the National Collaborative Research Infrastructure Strategy (NCRIS), Education Investment Fund, and other research infrastructure programs.

The Aperture Array Verification System (AAVS) is the SKA consortium’s testbed for SKA-LOW hardware. Suited close to the MWA core, AAVS is a single field-node of 256 SKALA-2 log-periodic antennas connected to a central power interface unit. It is operated by the LFAA Consortium under an MWA external instruments MoU.

The Engineering Development Array (EDA) is the MWA equivalent of a SKA-LOW field node, with 256 MWA dipole antennas arranged in the same pseudo-random pattern as AAVS. Signal conversion to fibre happens after the receiver (not in the antenna hub, as in the case of AAVS). The EDA is operated by Curtin University under an MWA external instrument MoU.

Report provided by Mia Walker