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.

The All-Sky Monitor on the Rossi X-ray Timing Explorer has been monitoring the sky in the 1.5 -12 keV band since late February. The instrument consists of three codedaperture cameras which can be rotated to view different regions by a motorized drive assembly. Intensities of ∼100 known sources are obtained via least-square fits of shadow patterns to the data and compiled to form x-ray light curves. Six orbital periodicities and four long-term periodicities, all previously known, have been detected in these light curves. Searches for additional sources have also been conducted. X-ray light curves for the Crab Nebula, Cyg X-1, 4U 1705-44, GRO J1655-40, and SMC X-1 are reported. They illustrate the quality of the results and the range of observed phenomena.

Millisecond pulsars are neutron stars that are thought to have been spun-up by mass accretion from a stellar companion.1 It is unknown whether there is a natural brake for this process, or if it continues until the centrifugal breakup limit is reached at submillisecond periods. Many neutron stars that are accreting mass from a companion star exhibit thermonuclear X-ray bursts that last tens of seconds, caused by unstable nuclear burning on their surfaces.2 Millisecond-period brightness oscillations during bursts from ten neutron stars (as distinct from other rapid X-ray variability that is also observed 3,4 ) are thought to measure the stellar spin, 2,5 but direct proof of a rotational origin has been lacking. Here, we report the detection of burst oscillations at the known spin frequency of an accreting millisecond pulsar, and we show that these oscillations always have the same rotational phase. This firmly establishes burst oscillations as nuclear-powered pulsations tracing the spin of accreting neutron stars, corroborating earlier evidence.5,6 The distribution of spin frequencies of the 11 nuclear-powered pulsars cuts off well below the breakup frequency for most neutron star models, supporting theoretical predictions that gravitational radiation losses can limit accretion torques in spinning up millisecond pulsars. 7-9The millisecond oscillations observed during X-ray bursts are not perfectly coherent, but usually drift in frequency by several hertz over the course of a burst, generally reaching an asymptotic maximium frequency that is repeatable in a given neutron star.2 This frequency drift has been interepreted as arising from angular momentum conservation in a decoupled surface burning layer that expands and contracts during the burst, so that the asymptotic frequency is the stellar spin frequency.10,11 A puzzle in this picture is why the oscillation persists late in the burst, well after the nuclear burning has spread over the entire star. Also, in most of these neutron stars, unexplained pairs of kilohertz quasi-periodic oscillations (kHz QPOs) are also observed in the non-burst X-ray emission, with the QPO separation frequency approximately equal to either the burst oscillation frequency or half this value, We have observed the transient X-ray source SAX J1808.4−3658, which has been detected in four outbursts since its discovery

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.