The Murchison Widefield Array (MWA) is one of three Square Kilometre Array Precursor telescopes and is located at the Murchison Radio-astronomy Observatory in the Murchison Shire of the mid-west of Western Australia, a location chosen for its extremely low levels of radio frequency interference. The MWA operates at low radio frequencies, 80-300 MHz, with a processed bandwidth of 30.72 MHz for both linear polarisations, and consists of 128 aperture arrays (known as tiles) distributed over a ß3-km diameter area. Novel hybrid hardware/software correlation and a real-time imaging and calibration systems comprise the MWA signal processing backend. In this paper, the as-built MWA is described both at a system and sub-system level, the expected performance of the array is presented, and the science goals of the instrument are summarised.

Astronomical widefield imaging of interferometric radio data is computationally expensive, especially for the large data volumes created by modern non-coplanar many-element arrays. We present a new widefield interferometric imager that uses the w-stacking algorithm and can make use of the w-snapshot algorithm. The performance dependencies of CASA's wprojection and our new imager are analysed and analytical functions are derived that describe the required computing cost for both imagers. On data from the Murchison Widefield Array, we find our new method to be an order of magnitude faster than w-projection, as well as being capable of full-sky imaging at full resolution and with correct polarisation correction. We predict the computing costs for several other arrays and estimate that our imager is a factor of 2-12 faster, depending on the array configuration. We estimate the computing cost for imaging the low-frequency Square-Kilometre Array observations to be 60 PetaFLOPS with current techniques. We find that combining w-stacking with the w-snapshot algorithm does not significantly improve computing requirements over pure w-stacking. The source code of our new imager is publicly released.

Using the Murchison Widefield Array (MWA), the low-frequency Square Kilometre Array (SKA1 LOW) precursor located in Western Australia, we have completed the GaLactic and Extragalactic All-sky MWA (GLEAM) survey, and present the resulting extragalactic catalogue, utilising the first year of observations. The catalogue covers 24, 831 square degrees, over declinations south of +30 • and Galactic latitudes outside 10 • of the Galactic plane, excluding some areas such as the Magellanic Clouds. It contains 307,455 radio sources with 20 separate flux density measurements across 72-231 MHz, selected from a time-and frequency-integrated image centred at 200 MHz, with a resolution of ≈ 2 . Over the catalogued region, we estimate that the catalogue is 90 % complete at 170 mJy, and 50 % complete at 55 mJy, and large areas are complete at even lower flux density levels. Its reliability is 99.97 % above the detection threshold of 5σ, which itself is typically 50 mJy. These observations constitute the widest fractional bandwidth and largest sky area survey at radio frequencies to date, and calibrate the low frequency flux density scale of the southern sky to better than 10 %. This paper presents details of the flagging, imaging, mosaicking, and source extraction/characterisation, as well as estimates of the completeness and reliability. All source measurements and images are available online . This is the first in a series of publications describing the GLEAM survey results.

The ‘drifting’ sub‐pulses exhibited by some radio pulsars have fascinated both observers and theorists for 30 years, and have been widely regarded as one of the most critical and potentially insightful aspects of their emission. Moreover, Ruderman & Sutherland, in their classic model, suggested that such regular modulation was produced by a system of sub‐beams, rotating around the magnetic axis under the action of E×B drift. Such ‘drift’ sequences have thus been thoroughly studied in a number of pulsars, but it has proven difficult to verify the rotating sub‐beam hypothesis, and thus to establish an illuminating connection between the phenomenon and the actual physics of the emission. Here, we report on detailed studies of pulsar B0943+10, whose nearly coherent sequences of ‘drifting’ sub‐pulses have permitted us to identify their origin as a system of sub‐beams that appear to circulate around the magnetic axis of the star. We introduce several new techniques of analysis, and we find that both the primary and secondary features in the fluctuation spectra of the star are aliases of their actual values. We have also developed a method of tracing the underlying pattern responsible for the observed sequences, using a ‘cartographic’ transform and its inverse, permitting us to study the characteristics of the polar emission ‘map’ and to confirm that such a ‘map’ in turn represents the observed sequence. We apply these techniques to the study of three different Arecibo observations: a 1992 430‐MHz sequence which includes a transition from the highly organized ‘B’ profile mode of the star to its disorganized ‘Q’ mode; a 1972 430‐MHz ‘B’‐mode sequence; and a 1990 111‐MHz ‘B’‐mode sequence. The ‘B’‐mode sequences are consistent in revealing that the emission pattern consists of 20 sub‐beams, which rotate around the magnetic axis in about 37 periods or 41 s. Even in the ‘Q’‐mode sequence, we find evidence of a compatible circulation time. The similarity of the sub‐beam patterns at different radio frequencies strongly suggests that the radiation is produced within a set of columns, which extend from close to the stellar surface up through the emission region and reflect some manner of ‘seeding’ phenomenon at their base. The sub‐beam emission is then tied neither to the stellar surface nor to the field. While the origin of the ‘memory’ responsible for the stability of the pattern over several circulation times is unknown, the hollow conical form of the average pattern is almost certainly the origin of the conal beam forms observed in most pulsars.

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