arXiv subjects

S. J. Tingay

Publications and source records attributed to S. J. Tingay.

At least 55 records · Page 3Linked to original sources

arXiv2019

A high time resolution study of the millisecond pulsar J2241-5236 at frequencies below 300 MHz

One of the major challenges for pulsar timing array (PTA) experiments is the mitigation of the effects of the turbulent interstellar medium (ISM) from timing data. These can potentially lead to measurable delays and/or distortions in the pulse profiles and scale strongly with the inverse of the radio frequency. Low-frequency observations are therefore highly appealing for characterizing them. However, in order to achieve the necessary time resolution to resolve profile features of short-period millisecond pulsars, phase-coherent de-dispersion is essential, especially at frequencies below $300$ MHz. We present the lowest-frequency ($80$-$220$ MHz), coherently de-dispersed detections of one of the most promising pulsars for current and future PTAs, PSR J2241$-$5236, using our new beam-former software for the MWA's voltage capture system (VCS), which reconstructs the time series at a much higher time resolution of $\sim 1 μ$s by re-synthesizing the recorded voltage data at $10$-kHz/$100$-$μ$s native resolutions. Our data reveal a dual-precursor type feature in the pulse profile that is either faint or absent in high-frequency observations from Parkes. The resultant high-fidelity detections have enabled dispersion measure (DM) determinations with very high precision, of the order of $(2$-$6)\times10^{-6}$ $\rm pc\,cm^{-3}$, owing to the microsecond level timing achievable for this pulsar at the MWA's low frequencies. This underscores the usefulness of low-frequency observations for probing the ISM toward PTA pulsars and informing optimal observing strategies for PTA experiments.

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arXiv2019

An International Survey of Front-End Receivers and Observing Performance of Telescopes for Radio Astronomy

This paper presents a survey of microwave front-end receivers installed at radio telescopes throughout the World. This unprecedented analysis was conducted as part of a review of front-end developments for Italian radio telescopes, initiated by the Italian National Institute for Astrophysics in 2016. Fifteen international radio telescopes have been selected to be representative of the instrumentation used for radio astronomical observations in the frequency domain from 300 MHz to 116 GHz. A comprehensive description of the existing receivers is presented and their characteristics are compared and discussed. The observing performances of the complete receiving chains are also presented. An overview of on-going developments illustrates and anticipates future trends in front-end projects to meet the most ambitious scientific research goals.

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arXiv2019

Multi-epoch Low Radio Frequency Surveys of the Kepler K2 Mission Campaign Fields 3, 4, and 5 with the Murchison Widefield Array

We present Murchison Widefield Array (MWA) monitoring of the Kepler K2 mission Fields 3, 4, and 5 at frequencies of 155 and 186 MHz, from observations contemporaneous with the K2 observations. This work follows from previous MWA and GMRT surveys of Field 1, with the current work benefiting from a range of improvements in the data processing and analysis. We continue to build a body of systematic low frequency blind surveys overlapping with transient/variable survey fields at other wavelengths, providing multi-wavelength data for object classes such as flare stars. From the current work, we detect no variable objects at a surface density above 2e-4 per square degree, at flux densities of ~500 mJy, and observation cadence of days to weeks, representing almost an order of magnitude decrease in measured upper limits compared to previous results in this part of observational parameter space. This continues to show that radio transients at metre and centimetre wavelengths are rare.

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arXiv2019

Gridded and direct Epoch of Reionisation bispectrum estimates using the Murchison Widefield Array

We apply two methods to estimate the 21~cm bispectrum from data taken within the Epoch of Reionisation (EoR) project of the Murchison Widefield Array (MWA). Using data acquired with the Phase II compact array allows a direct bispectrum estimate to be undertaken on the multiple redundantly-spaced triangles of antenna tiles, as well as an estimate based on data gridded to the $uv$-plane. The direct and gridded bispectrum estimators are applied to 21 hours of high-band (167--197~MHz; $z$=6.2--7.5) data from the 2016 and 2017 observing seasons. Analytic predictions for the bispectrum bias and variance for point source foregrounds are derived. We compare the output of these approaches, the foreground contribution to the signal, and future prospects for measuring the bispectra with redundant and non-redundant arrays. We find that some triangle configurations yield bispectrum estimates that are consistent with the expected noise level after 10 hours, while equilateral configurations are strongly foreground-dominated. Careful choice of triangle configurations may be made to reduce foreground bias that hinders power spectrum estimators, and the 21~cm bispectrum may be accessible in less time than the 21~cm power spectrum for some wave modes, with detections in hundreds of hours.

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arXiv2019

Robust statistics toward detection of the 21 cm signal from the Epoch of Reionisation

We explore methods for robust estimation of the 21 cm signal from the Epoch of Reionisation (EoR). A Kernel Density Estimator (KDE) is introduced for measuring the spatial temperature fluctuation power spectrum from the EoR. The KDE estimates the underlying probability distribution function of fluctuations as a function of spatial scale, and contains different systematic biases and errors to the typical approach to estimating the fluctuation power spectrum. Extraction of histograms of visibilities allows moments analysis to be used to discriminate foregrounds from 21 cm signal and thermal noise. We use the information available in the histograms, along with the statistical dis-similarity of foregrounds from two independent observing fields, to robustly separate foregrounds from cosmological signal, while making no assumptions about the Gaussianity of the signal. Using two independent observing fields to robustly discriminate signal from foregrounds is crucial for the analysis presented in this paper. We apply the techniques to 13 hours of Murchison Widefield Array (MWA) EoR data over two observing fields. We compare the output to that obtained with a comparative power spectrum estimation method, and demonstrate the reduced foreground contamination using this approach. Using the second moment obtained directly from the KDE distribution functions yields a factor of 2-3 improvement in power for k < 0.3hMpc^{-1} compared with a matched delay space power estimator, while weighting data by additional statistics does not offer significant improvement beyond that available for thermal noise-only weights.

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arXiv2018

Hunting for radio emission from the intermittent pulsar J1107-5907 at low frequencies

The rare intermittent pulsars pose some of the most challenging questions surrounding the pulsar emission mechanism, but typically have relatively minimal low-frequency ($\lesssim$ 300 MHz) coverage. We present the first low-frequency detection of the intermittent pulsar J1107-5907 with the Murchison Widefield Array (MWA) at 154 MHz and the simultaneous detection from the recently upgraded Molonglo Observatory Synthesis Telescope (UTMOST) at 835 MHz, as part of an on-going observing campaign. During a 30-minute simultaneous observation, we detected the pulsar in its bright emission state for approximately 15 minutes, where 86 and 283 pulses were detected above a signal-to-noise threshold of 6 with the MWA and UTMOST, respectively. Of the detected pulses, 51 had counterparts at both frequencies and exhibited steep spectral indices for both the bright main pulse component and the precursor component. We find that the bright state pulse energy distribution is best parameterised by a log-normal distribution at both frequencies, contrary to previous results which suggested a power law distribution. Further low-frequency observations are required in order to explore in detail aspects such as pulse-to-pulse variability, intensity modulations and to better constrain the signal propagation effects due to the interstellar medium and intermittency characteristics at these frequencies. The spectral index, extended profile emission covering a large fraction of pulse longitude, and the broadband intermittency of PSR J1107-5907 suggests that future low-frequency pulsar searches, for instance those planned with SKA-Low, will be in an excellent position to find and investigate new pulsars of this type.

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arXiv2018

The Murchison Widefield Array Transients Survey (MWATS). A search for low frequency variability in a bright Southern hemisphere sample

We report on a search for low-frequency radio variability in 944 bright (> 4Jy at 154 MHz) unresolved, extragalactic radio sources monitored monthly for several years with the Murchison Widefield Array. In the majority of sources we find very low levels of variability with typical modulation indices < 5%. We detect 15 candidate low frequency variables that show significant long term variability (>2.8 years) with time-averaged modulation indices M = 3.1 - 7.1%. With 7/15 of these variable sources having peaked spectral energy distributions, and only 5.7% of the overall sample having peaked spectra, we find an increase in the prevalence of variability in this spectral class. We conclude that the variability seen in this survey is most probably a consequence of refractive interstellar scintillation and that these objects must have the majority of their flux density contained within angular diameters less than 50 milli-arcsec (which we support with multi-wavelength data). At 154 MHz we demonstrate that interstellar scintillation time-scales become long (~decades) and have low modulation indices, whilst synchrotron driven variability can only produce dynamic changes on time-scales of hundreds of years, with flux density changes less than one milli-jansky (without relativistic boosting). From this work we infer that the low frequency extra-galactic southern sky, as seen by SKA-Low, will be non-variable on time-scales shorter than one year.

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arXiv2018

No low-frequency emission from extremely bright Fast Radio Bursts

We present the results of a coordinated campaign conducted with the Murchison Widefield Array (MWA) to shadow Fast Radio Bursts (FRBs) detected by the Australian Square Kilometre Array Pathfinder (ASKAP) at 1.4 GHz, which resulted in simultaneous MWA observations of seven ASKAP FRBs. We de-dispersed the $24$ $\times$ $1.28$ MHz MWA images across the $170-200$ MHz band taken at 0.5 second time resolution at the known dispersion measures (DMs) and arrival times of the bursts and searched both within the ASKAP error regions (typically $\sim$ $10$ arcmin $\times$ $10$ arcmin), and beyond ($4$ deg $\times$ $4$ deg). We identified no candidates exceeding a $5σ$ threshold at these DMs in the dynamic spectra. These limits are inconsistent with the mean fluence scaling of $α=-1.8 \pm 0.3$ (${\cal F}_ν\propto ν^α$, where $ν$ is the observing frequency) that is reported for ASKAP events, most notably for the three high fluence (${\cal F}_{1.4\,{\rm GHz}} \gtrsim 100$ Jy ms) FRBs 171020, 180110 and 180324. Our limits show that pulse broadening alone cannot explain our non-detections, and that there must be a spectral turnover at frequencies above 200 MHz. We discuss and constrain parameters of three remaining plausible spectral break mechanisms: free-free absorption, intrinsic spectral turn-over of the radiative processes, and magnification of signals at ASKAP frequencies by caustics or scintillation. If free-free absorption were the cause of the spectral turnover, we constrain the thickness of the absorbing medium in terms of the electron temperature, $T$, to $< 0.03$ $(T/10^4 K)^{-1.35}$ pc for FRB 171020.

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arXiv2018

Interplanetary Scintillation with the Murchison Widefield Array V: An All-sky Survey of Compact Sources using a Modern Low-frequency Radio Telescope

We describe the parameters of a low-frequency all-sky survey of compact radio sources using Interplanetary Scintillation (IPS), undertaken with the Murchison Widefield Array (MWA). While this survey gives important complementary information to low-resolution survey such as the MWA GLEAM survey, providing information on the subarsecond structure of every source, a survey of this kind has not been attempted in the era of low-frequency imaging arrays such as the MWA and LOFAR. Here we set out the capabilities of such a survey, describing the limitations imposed by the heliocentric observing geometry and by the instrument itself. We demonstrate the potential for IPS measurements at any point on the celestial sphere and we show that at 160MHz, reasonable results can be obtained within 30deg of the ecliptic (2π str: half the sky). We also suggest some observational strategies and describe the first such survey, the MWA Phase I IPS survey. Finally we analyse the potential of the recently-upgraded MWA and discuss the potential of the SKA-low to use IPS to probe sub-mJy flux density levels at sub-arcsecond angular resolution.

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arXiv2018

Assessment of ionospheric activity tolerances for Epoch of Reionisation science with the Murchison Widefield Array

Structure imprinted in foreground extragalactic point sources by ionospheric refraction has the potential to contaminate Epoch of Reionisation (EoR) power spectra of the 21~cm emission line of neutral hydrogen. The alteration of the spatial and spectral structure of foreground measurements due to total electron content (TEC) gradients in the ionosphere create a departure from the expected sky signal. We present a general framework for understanding the signatures of ionospheric behaviour in the two-dimensional (2D) neutral hydrogen power spectrum measured by a low-frequency radio interferometer. Two primary classes of ionospheric behaviour are considered, corresponding to dominant modes observed in Murchison Widefield Array (MWA) EoR data; namely, anisotropic structured wave behaviour, and isotropic turbulence. Analytic predictions for power spectrum bias due to this contamination are computed, and compared with simulations. We then apply the ionospheric metric described in Jordan et al. (2017) to study the impact of ionospheric structure on MWA data, by dividing MWA EoR datasets into classes with good and poor ionospheric conditions, using sets of matched 30-minute observations from 2014 September. The results are compared with the analytic and simulated predictions, demonstrating the observed bias in the power spectrum when the ionosphere is active (displays coherent structures or isotropic turbulence). The analysis demonstrates that unless ionospheric activity can be quantified and corrected, active data should not be included in EoR analysis in order to avoid systematic biases in cosmological power spectra. When data are corrected with a model formed from the calibration information, bias reduces below the expected 21~cm signal level. Data are considered `quiet' when the median measured source position offsets are less than 10-15~arcseconds.

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arXiv2018

Measuring the global 21-cm signal with the MWA-I: improved measurements of the Galactic synchrotron background using lunar occultation

We present early results from a project to measure the sky-averaged (global), redshifted $21\,$cm signal from the Epoch of Reionisation (EoR), using the Murchison Widefield Array (MWA) telescope. Because interferometers are not sensitive to a spatially-invariant global average, they cannot be used to detect this signal using standard techniques. However, lunar occultation of the radio sky imprints a spatial structure on the global signal, allowing us to measure the average brightness temperature of the patch of sky immediately surrounding the Moon. In this paper we present one night of Moon observations with the MWA between 72 - 230 MHz and verify our techniques to extract the background sky temperature from measurements of the Moon's flux density. We improve upon previous work using the lunar occultation technique by using a more sophisticated model for reflected `earthshine' and by employing image differencing to remove imaging artefacts. We leave the Moon's (constant) radio brightness temperature as a free parameter in our fit to the data and as a result, measure $T_{\rm{moon}} = 180 \pm 12 $ K and a Galactic synchrotron spectral index of $-2.64\pm0.14$, at the position of the Moon. Finally, we evaluate the prospects of the lunar occultation technique for a global EoR detection and map out a way forward for future work with the MWA.

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arXiv2018

In situ measurement of MWA primary beam variation using ORBCOMM

We provide the first in situ measurements of antenna element (tile) beam shapes of the Murchison Widefield Array (MWA), a low radio-frequency interferometer and an SKA precursor. Most current MWA processing pipelines use an assumed beam shape, errors in which can cause absolute and relative flux density errors, as well as polarisation 'leakage'. This makes understanding the primary beam of paramount importance, especially for sensitive experiments such as a measurement of the 21 cm line from the epoch of reionisation (EoR). The calibration requirements for measuring the EoR 21 cm line are so extreme that tile to tile beam variations may affect our ability to make a detection. Measuring the primary beam shape from visibilities alone is challenging, as multiple instrumental, atmospheric, and astrophysical factors contribute to uncertainties in the data. Building on the methods of Neben et al. (2015), we tap directly into the receiving elements of the MWA before any digitisation or correlation of the signal. Using ORBCOMM satellite passes we are able to produce all-sky maps for 4 separate tiles in the XX polarisation. We find good agreement with the cutting-edge 'fully' embedded element (FEE) model of Sokolowski et al. (2017), and observe that the MWA beamformers consistently recreate beam shapes to within ~1dB in the reliable areas of our beam maps. We also clearly observe the effects of a missing dipole from a tile in one of our beam maps, and show that the FEE model is able to reproduce this modified beam shape. We end by motivating and outlining additional onsite experiments to further constrain the primary beam behaviour.

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arXiv2018

Observations of Low-Frequency Radio Emission from Millisecond Pulsars and Multipath Propagation in the Interstellar Medium

Studying the gravitational-wave sky with pulsar timing arrays (PTAs) is a key science goal for the Square Kilometre Array (SKA) and its pathfinder telescopes. With current PTAs reaching sub-microsecond timing precision, making accurate measurements of interstellar propagation effects and mitigating them effectively has become increasingly important to realise PTA goals. As these effects are much stronger at longer wavelengths, low-frequency observations are most appealing for characterizing the interstellar medium (ISM) along the sight lines toward PTA pulsars. The Murchison Widefield Array (MWA) and the Engineering Development Array (EDA), which utilizes MWA technologies, present promising opportunities for undertaking such studies, particularly for PTA pulsars located in the southern sky. Such pulsars are also the prime targets for PTA efforts planned with the South African MeerKAT, and eventually with the SKA. In this paper we report on observations of two bright southern millisecond pulsars PSRs J0437-4715 and J2145-0750 made with these facilities; MWA observations sampling multiple frequencies across the 80-250 MHz frequency range, while the EDA providing direct-sampled baseband data to yield a large instantaneous usable bandwidth of $\sim$200 MHz. Using these observations, we investigate various aspects relating to pulsar emission and ISM properties, such as spectral evolution of the mean pulse shape, scintillation as a function of frequency, chromaticity in interstellar dispersion, and flux density spectra at low frequencies. Systematic and regular monitoring observations will help ascertain the role of low-frequency measurements in PTA experiments, while simultaneously providing a detailed characterization of the ISM toward the pulsars, which will be useful in devising optimal observing strategies for future PTA experiments.

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arXiv2018

Comparing Redundant and Sky Model Based Interferometric Calibration: A First Look with Phase II of the MWA

Interferometric arrays seeking to measure the 21 cm signal from the Epoch of Reionization must contend with overwhelmingly bright emission from foreground sources. Accurate recovery of the 21 cm signal will require precise calibration of the array, and several new avenues for calibration have been pursued in recent years, including methods using redundancy in the antenna configuration. The newly upgraded Phase II of Murchison Widefield Array (MWA) is the first interferometer that has large numbers of redundant baselines while retaining good instantaneous UV-coverage. This array therefore provides a unique opportunity to compare redundant calibration with sky-model based algorithms. In this paper, we present the first results from comparing both calibration approaches with MWA Phase II observations. For redundant calibration, we use the package OMNICAL, and produce sky-based calibration solutions with the analysis package Fast Holographic Deconvolution (FHD). There are three principal results. (1) We report the success of OMNICAL on observations of ORBComm satellites, showing substantial agreement between redundant visibility measurements after calibration. (2) We directly compare OMNICAL calibration solutions with those from FHD, and demonstrate these two different calibration schemes give extremely similar results. (3) We explore improved calibration by combining OMNICAL and FHD. We evaluate these combined methods using power spectrum techniques developed for EoR analysis and find evidence for marginal improvements mitigating artifacts in the power spectrum. These results are likely limited by signal-to-noise in the six hours of data used, but suggest future directions for combining these two calibration schemes.

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arXiv2018

Galactic synchrotron distribution derived from 152 HII region absorption features in the full GLEAM survey

We derive the synchrotron distribution in the Milky Way disk from HII region absorption observations over -40° < l < 40° at six frequencies of 76.2, 83.8, 91.5, 99.2, 106.9, and 114.6 MHz with the GaLactic and Extragalactic All-sky Murchison widefield array survey (GLEAM). We develop a new method of emissivity calculation by taking advantage of the Haslam et al., (1981) map and known spectral indices, which enable us to simultaneously derive the emissivity and the optical depth of HII regions at each frequency. We show our derived synchrotron emissivities based on 152 absorption features of HII regions using both the method previously adopted in the literature and our improved method. We derive the synchrotron emissivity from HII regions to the Galactic edge along the line of sight and, for the first time, derive the emissivity from HII regions to the Sun. These results provide direct information on the distribution of the Galactic magnetic field and cosmic-ray electrons for future modelling.

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arXiv2018

Limits on radio emission from meteors using the MWA

Recently, low frequency, broadband radio emission has been observed accompanying bright meteors by the Long Wavelength Array (LWA). The broadband spectra between 20 and 60 MHz were captured for several events, while the spectral index (dependence of flux density on frequency, with $S_ν\propto ν^α$) was estimated to be $-4\pm1$ during the peak of meteor afterglows. Here we present a survey of meteor emission and other transient events using the Murchison Widefield Array (MWA) at 72-103 MHz. In our 322-hour survey, down to a $5σ$ detection threshold of 3.5 Jy/beam, no transient candidates were identified as intrinsic emission from meteors. We derived an upper limit of -3.7 (95% confidence limit) on the spectral index in our frequency range. We also report detections of other transient events, like reflected FM broadcast signals from small satellites, conclusively demonstrating the ability of the MWA to detect and track space debris on scales as small as 0.1 m in low Earth orbits.

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arXiv2018

Extended X-ray emission in PKS 1718$-$649

PKS 1718$-$649 is one of the closest and most comprehensively studied candidates of a young active galactic nucleus (AGN) that is still embedded in its optical host galaxy. The compact radio structure, with a maximal extent of a few parsecs, makes it a member of the group of compact symmetric objects (CSO). Its environment imposes a turnover of the radio synchrotron spectrum towards lower frequencies, also classifying PKS 1718$-$649 as gigahertz-peaked radio spectrum (GPS) source. Its close proximity has allowed the first detection of extended X-ray emission in a GPS/CSO source with Chandra that is for the most part unrelated to nuclear feedback. However, not much is known about the nature of this emission. By co-adding all archival Chandra data and complementing these datasets with the large effective area of XMM-Newton, we are able to study the detailed physics of the environment of PKS 1718$-$649. Not only can we confirm that the bulk of the $\lesssim$kiloparsec-scale environment emits in the soft X-rays, but we also identify the emitting gas to form a hot, collisionally ionized medium. While the feedback of the central AGN still seems to be constrained to the inner few parsecs, we argue that supernovae are capable of producing the observed large-scale X-ray emission at a rate inferred from its estimated star formation rate.

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arXiv2018

A Serendipitous MWA Search for Narrow-band and Broad-band Low Frequency Radio Transmissions from 1I/2017 U1 'Oumuamua

We examine data from the Murchison Widefield Array (MWA) in the frequency range 72 -- 102 MHz for a field-of-view that serendipitously contained the interstellar object 'Oumuamua on 2017 November 28. Observations took place with time resolution of 0.5 s and frequency resolution of 10 kHz. %This observation was undertaken for another purpose but due to the MWA's extremely large field-of-view, 'Oumuamua was serendipitously observed simultaneously. Based on the interesting but highly unlikely suggestion that 'Oumuamua is an interstellar spacecraft, due to some unusual orbital and morphological characteristics, we examine our data for signals that might indicate the presence of intelligent life associated with 'Oumuamua. We searched our radio data for: 1) impulsive narrow-band signals; 2) persistent narrow-band signals; and 3) impulsive broadband signals. We found no such signals with non-terrestrial origins and make estimates of the upper limits on Equivalent Isotropic Radiated Power (EIRP) for these three cases of approximately 7 kW, 840 W, and 100 kW, respectively. These transmitter powers are well within the capabilities of human technologies, and are therefore plausible for alien civilizations. While the chances of positive detection in any given Search for Extraterrestrial Intelligence (SETI) experiment are vanishingly small, the characteristics of new generation telescopes such as the MWA (and in the future, the Square Kilometre Array) make certain classes of SETI experiment easy, or even a trivial by-product of astrophysical observations. This means that the future costs of SETI experiments are very low, allowing large target lists to partially balance the low probability of a positive detection.

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