A. Stewart
A5005805808- Radio Astronomy Observations and Technology
- Astrophysics and Cosmic Phenomena
- Pulsars and Gravitational Waves Research
- Gamma-ray bursts and supernovae
- Galaxies: Formation, Evolution, Phenomena
- Astronomical Observations and Instrumentation
- Geophysics and Gravity Measurements
- Astrophysical Phenomena and Observations
- Astronomy and Astrophysical Research
- GNSS positioning and interference
- Soil Moisture and Remote Sensing
- Particle accelerators and beam dynamics
- Adaptive optics and wavefront sensing
- Particle Accelerators and Free-Electron Lasers
- Stellar, planetary, and galactic studies
- Geophysics and Sensor Technology
- Solar and Space Plasma Dynamics
- Scientific Research and Discoveries
- Dark Matter and Cosmic Phenomena
- Radio Wave Propagation Studies
- Antenna Design and Optimization
- Advanced Optical Sensing Technologies
- Relativity and Gravitational Theory
- Infrared Target Detection Methodologies
- Advances in Oncology and Radiotherapy
The University of Sydney
2015-2024
University of Oxford
1973-2020
University of Southampton
2015-2018
Laboratory for Atmospheric and Space Physics
2016
National Institute for Astrophysics
2015
Joint Institute for VLBI ERIC
2015
SKA Telescope, Australia
2015
Science Oxford
1953-2015
National Centre for Radio Astrophysics
2014
The Rapid ASKAP Continuum Survey (RACS) is the first large sky survey using Australian Square Kilometre Array Pathfinder (ASKAP), covering south of +41$^\circ$ declination. With ASKAP's large, instantaneous field view, $\sim 31$ deg$^2$, RACS observed entire at a central frequency 887.5 MHz 903 individual pointings with 15 minute observations. This has resulted in deepest radio full Southern to date these frequencies. In this paper, we present Stokes I catalogue derived from survey. was...
Abstract We present a search for extragalactic fast blue optical transients (FBOTs) during Phase I of the Zwicky Transient Facility (ZTF). identify 38 candidates with durations above half-maximum light 1 day < t 1/2 12 days, which 28 have ( g − r ≲ −0.2 mag) colors at peak light. Of (28 FBOTs), 19 (13) can be spectroscopically classified as core-collapse supernovae (SNe): 11 (8) H- or He-rich (Type II/IIb/Ib) SNe, 6 (4) interacting IIn/Ibn) and 2 (1) H&He-poor Ic/Ic-BL) SNe. Two FBOTs...
Context. The LOw Frequency ARray (LOFAR) radio telescope is a giant digital phased array interferometer with multiple antennas distributed in Europe. It provides discrete sets of Fourier components the sky brightness. Recovering original brightness distribution aperture synthesis forms an inverse problem that can be solved by various deconvolution and minimization methods Aims. Recent papers have established clear link between nature interferometry measurement "compressed sensing" (CS)...
We present the Multifrequency Snapshot Sky Survey (MSSS), first northern-sky LOFAR imaging survey. In this introductory paper, we describe in detail motivation and design of Compared to previous radio surveys, MSSS is exceptional due its intrinsic multifrequency nature providing information about spectral properties detected sources over more than two octaves (from 30 160 MHz). The broadband frequency coverage, together with fast survey speed generated by LOFAR's multibeaming capabilities,...
We present the highest-quality polarisation profiles to date of 16 non-recycled pulsars and four millisecond pulsars, observed below 200 MHz with LOFAR high-band antennas. Based on profiles, we perform an initial investigation expected observational effects resulting from propagation polarised emission in pulsar magnetosphere interstellar medium. The predictions magnetospheric birefringence have been tested using spectra pulse width fractional multifrequency data. derived offer only partial...
This study aims to characterise the polarized foreground emission in ELAIS-N1 field and address its possible implications for extraction of cosmological 21-cm signal from Low-Frequency Array - Epoch Reionization (LOFAR-EoR) data. We use high band antennas LOFAR image this region RM-synthesis unravel structures at Galactic latitudes. The brightness temperature detected is on average 4 K intensity covers range -10 +13rad m^-2 Faraday depth. total polarization angle show a wide morphological...
We present the results of a four-month campaign searching for low-frequency radio transients near North Celestial Pole with Low-Frequency Array (LOFAR), as part Multifrequency Snapshot Sky Survey (MSSS). The data were recorded between 2011 December and 2012 April comprised 2149 11-minute snapshots, each covering 175 deg^2. have found one convincing candidate astrophysical transient, duration few minutes flux density at 60 MHz 15-25 Jy. transient does not repeat has no obvious optical or...
The low-frequency radio spectra of the hotspots within powerful galaxies can provide valuable information about physical processes operating at site jet termination. These are responsible for dissipation kinetic energy, particle acceleration, and magnetic-field generation. Here, we report new observations galaxy Cygnus A using Low Frequency Array (LOFAR) between 109 183 MHz, an angular resolution ∼3.5 arcsec. emission lobes is found to have a complex spectral index distribution, with...
Due to their steep spectra, low-frequency observations of FR II radio galaxies potentially provide key insights in the morphology, energetics and spectrum these powerful sources. However, limitations imposed by previous generation interferometers at metre wavelengths has meant that this region parameter space remains largely unexplored. In paper, first a series examining IIs low frequencies, we use LOFAR between 50 160 MHz, along with complementary archival X-ray data, explore properties two...
Abstract The Variables and Slow Transients Survey (VAST) on the Australian Square Kilometre Array Pathfinder (ASKAP) is designed to detect highly variable transient radio sources timescales from 5 s $\sim\!5$ yr. In this paper, we present survey description, observation strategy initial results VAST Phase I Pilot Survey. This pilot consists of $\sim\!162$ h observations conducted at a central frequency 888 MHz between 2019 August 2020 August, with typical rms sensitivity $0.24\ \mathrm{mJy\...
We present results from a circular polarisation survey for radio stars in the Rapid ASKAP Continuum Survey (RACS). RACS is of entire sky south $\delta=+41^\circ$ being conducted with Australian Square Kilometre Array Pathfinder telescope (ASKAP) over 288 MHz wide band centred on 887.5 MHz. The data we analyse includes Stokes I and V products to an RMS sensitivity 250 $\mu$Jy PSF$^{-1}$. searched sources fractional above 6 per cent, after excluding imaging artefacts, leakage, known pulsars...
The Sun is an active source of radio emission which often associated with energetic phenomena such as solar flares and coronal mass ejections (CMEs). At low frequencies (<100 MHz), the has not been imaged extensively because instrumental limitations previous telescopes. Here, combined high spatial, spectral temporal resolution Low Frequency Array (LOFAR) was used to study Type III bursts at 30-90 MHz their association CMEs. 126 simultaneous tied-array beams within 5 radii centre. This method...
Active M-dwarfs are known to produce bursty radio emission, and multi-wavelength studies have shown that Solar-like magnetic activity occurs in these stars. However, coherent bursts from active often been difficult interpret the Solar paradigm. We present Australian Square Array Pathfinder (ASKAP) observations of UV Ceti at a central frequency 888 MHz. detect several periodic, pulses occurring over timescale consistent with rotational period Ceti. The properties pulsed emission show they...
Context. LOFAR offers the unique capability of observing pulsars across 10−240 MHz frequency range with a fractional bandwidth roughly 50%. This spectral is well suited for studying evolution pulse profile morphology caused by both intrinsic and extrinsic effects such as changing emission altitude in pulsar magnetosphere or scatter broadening interstellar medium, respectively.
PSR B0823+26, a 0.53-s radio pulsar, displays host of emission phenomena over time-scales seconds to (at least) hours, including nulling, subpulse drifting, and mode-changing. Studying pulsars like B0823+26 provides further insight into the relationship between these various what they might teach us about pulsar magnetospheres. Here we report on LOFAR (Low-Frequency Array) discovery that has weak sporadically emitting ‘quiet’ (Q) mode is 100 times weaker (on average) nulling fraction...
We present a deep, low-frequency radio continuum study of the nearby Fanaroff--Riley class I (FR I) galaxy 3C 31 using combination LOw Frequency ARray (LOFAR; 30--85 and 115--178 MHz), Very Large Array (VLA; 290--420 Westerbork Synthesis Radio Telescope (WSRT; 609 MHz) Giant Metre (GMRT; 615 observations. Our new LOFAR 145-MHz map shows that has largest physical size $1.1$ Mpc in projection, which means now falls giant galaxies. model intensities with advective cosmic-ray transport, evolving...
We report on the results of a search for radio transients between 115 and 190 MHz with LOw-Frequency ARray (LOFAR). Four fields have been monitored cadences 15 min several months. A total 151 images were obtained, giving survey area 2275 deg2. analysed our data using standard LOFAR tools searched Transients Pipeline. No credible transient candidate has detected; however, we are able to set upper limits surface density sources at low frequencies. also show that low-frequency surveys more...