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In this paper we describe the system design and capabilities of the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope at the conclusion of its construction project and commencement of science operations. ASKAP is one of the first radio telescopes to deploy phased array feed (PAF) technology on a large scale, giving it an instantaneous field of view that covers 31 square degrees at 800 MHz. As a two-dimensional array of 36x12m antennas, with baselines ranging from 22m to 6km, ASKAP also has excellent snapshot imaging capability and 10 arcsecond resolution. This, combined with 288 MHz of instantaneous bandwidth and a unique third axis of rotation on each antenna, gives ASKAP the capability to create high dynamic range images of large sky areas very quickly. It is an excellent telescope for surveys between 700 MHz and 1800 MHz and is expected to facilitate great advances in our understanding of galaxy formation, cosmology and radio transients while opening new parameter space for discovery of the unknown., Comment: 38 pages, 24 figures, accepted for publication in PASA

We describe the performance of the Boolardy Engineering Test Array (BETA), the prototype for the Australian Square Kilometre Array Pathfinder telescope ASKAP. BETA is the first aperture synthesis radio telescope to use phased array feed technology, giving it the ability to electronically form up to nine dual-polarization beams. We report the methods developed for forming and measuring the beams, and the adaptations that have been made to the traditional calibration and imaging procedures in order to allow BETA to function as a multi-beam aperture synthesis telescope. We describe the commissioning of the instrument and present details of BETA's performance: sensitivity, beam characteristics, polarimetric properties and image quality. We summarise the astronomical science that it has produced and draw lessons from operating BETA that will be relevant to the commissioning and operation of the final ASKAP telescope., Comment: Accepted for publication in PASA

The Boolardy Engineering Test Array is a 6 x 12 m dish interferometer and the prototype of the Australian Square Kilometre Array Pathfinder (ASKAP), equipped with the first generation of ASKAP's phased array feed (PAF) receivers. These facilitate rapid wide-area imaging via the deployment of simultaneous multiple beams within a 30 square degree field of view. By cycling the array through 12 interleaved pointing positions and using 9 digitally formed beams we effectively mimic a traditional 1 hour x 108 pointing survey, covering 150 square degrees over 711 - 1015 MHz in 12 hours of observing time. Three such observations were executed over the course of a week. We verify the full bandwidth continuum imaging performance and stability of the system via self-consistency checks and comparisons to existing radio data. The combined three epoch image has arcminute resolution and a 1-sigma thermal noise level of 375 micro-Jy per beam, although the effective noise is a factor 3 higher due to residual sidelobe confusion. From this we derive a catalogue of 3,722 discrete radio components, using the 35 percent fractional bandwidth to measure in-band spectral indices for 1,037 of them. A search for transient events reveals one significantly variable source within the survey area. The survey covers approximately two-thirds of the Spitzer South Pole Telescope Deep Field. This pilot project demonstrates the viability and potential of using PAFs to rapidly and accurately survey the sky at radio wavelengths., Comment: 20 pages, 13 figures, 4 tables. Accepted for publication in MNRAS

We use observations from the Boolardy Engineering Test Array (BETA) of the Australian Square Kilometre Array Pathfinder (ASKAP) telescope to search for transient radio sources in the field around the intermittent pulsar PSR J1107-5907. The pulsar is thought to switch between an "off" state in which no emission is detectable, a weak state and a strong state. We ran three independent transient detection pipelines on two-minute snapshot images from a 13 hour BETA observation in order to 1) study the emission from the pulsar, 2) search for other transient emission from elsewhere in the image and 3) to compare the results from the different transient detection pipelines. The pulsar was easily detected as a transient source and, over the course of the observations, it switched into the strong state three times giving a typical timescale between the strong emission states of 3.7 hours. After the first switch it remained in the strong state for almost 40 minutes. The other strong states lasted less than 4 minutes. The second state change was confirmed using observations with the Parkes radio telescope. No other transient events were found and we place constraints on the surface density of such events on these timescales. The high sensitivity Parkes observations enabled us to detect individual bright pulses during the weak state and to study the strong state over a wide observing band. We conclude by showing that future transient surveys with ASKAP will have the potential to probe the intermittent pulsar population., Comment: Accepted by MNRAS

We present HI imaging of the galaxy group IC 1459 carried out with six antennas of the Australian SKA Pathfinder equipped with phased-array feeds. We detect and resolve HI in eleven galaxies down to a column density of $\sim10^{20}$ cm$^{-2}$ inside a ~6 deg$^2$ field and with a resolution of ~1 arcmin on the sky and ~8 km/s in velocity. We present HI images, velocity fields and integrated spectra of all detections, and highlight the discovery of three HI clouds -- two in the proximity of the galaxy IC 5270 and one close to NGC 7418. Each cloud has an HI mass of $10^9$ M$_\odot$ and accounts for ~15% of the HI associated with its host galaxy. Available images at ultraviolet, optical and infrared wavelengths do not reveal any clear stellar counterpart of any of the clouds, suggesting that they are not gas-rich dwarf neighbours of IC 5270 and NGC 7418. Using Parkes data we find evidence of additional extended, low-column-density HI emission around IC 5270, indicating that the clouds are the tip of the iceberg of a larger system of gas surrounding this galaxy. This result adds to the body of evidence on the presence of intra-group gas within the IC 1459 group. Altogether, the HI found outside galaxies in this group amounts to several times $10^9$ M$_\odot$, at least 10% of the HI contained inside galaxies. This suggests a substantial flow of gas in and out of galaxies during the several billion years of the group's evolution., Comment: MNRAS accepted

We report the discovery of a new 21-cm HI absorption system using commissioning data from the Boolardy Engineering Test Array of the Australian Square Kilometre Array Pathfinder (ASKAP). Using the 711.5 - 1015.5 MHz band of ASKAP we were able to conduct a blind search for the 21-cm line in a continuous redshift range between $z = 0.4$ and 1.0, which has, until now, remained largely unexplored. The absorption line is detected at $z = 0.44$ towards the GHz-peaked spectrum radio source PKS B1740$-$517 and demonstrates ASKAP's excellent capability for performing a future wide-field survey for HI absorption at these redshifts. Optical spectroscopy and imaging using the Gemini-South telescope indicates that the HI gas is intrinsic to the host galaxy of the radio source. The narrow OIII emission lines show clear double-peaked structure, indicating either large-scale outflow or rotation of the ionized gas. Archival data from the \emph{XMM-Newton} satellite exhibit an absorbed X-ray spectrum that is consistent with a high column density obscuring medium around the active galactic nucleus. The HI absorption profile is complex, with four distinct components ranging in width from 5 to 300 km s$^{-1}$ and fractional depths from 0.2 to 20 per cent. In addition to systemic HI gas, in a circumnuclear disc or ring structure aligned with the radio jet, we find evidence for a possible broad outflow of neutral gas moving at a radial velocity of $v \sim 300$ km s$^{-1}$. We infer that the expanding young radio source ($t_{\rm age} \approx 2500$ yr) is cocooned within a dense medium and may be driving circumnuclear neutral gas in an outflow of $\sim$ 1 $\mathrm{M}_{\odot}$ yr$^{-1}$., Comment: 18 pages, 11 figures, accepted for publication in MNRAS

This paper describes the system architecture of a newly constructed radio telescope - the Boolardy Engineering Test Array, which is a prototype of the Australian Square Kilometre Array Pathfinder telescope. Phased array feed technology is used to form multiple simultaneous beams per antenna, providing astronomers with unprecedented survey speed. The test array described here is a 6-antenna interferometer, fitted with prototype signal processing hardware capable of forming at least 9 dual-polarisation beams simultaneously, allowing several square degrees to be imaged in a single pointed observation. The main purpose of the test array is to develop beamforming and wide-field calibration methods for use with the full telescope, but it will also be capable of limited early science demonstrations., Comment: 17 pages, 12 figures, accepted for publication in PASA