ABSTRACT
We present sofia 2, the fully automated 3D source finding pipeline for the WALLABY extragalactic H i survey with the Australian SKA Pathfinder (ASKAP). sofia 2 is a reimplementation of parts ...of the original sofia pipeline in the c programming language and makes use of OpenMP for multithreading of the most time-critical algorithms. In addition, we have developed a parallel framework called sofia-X that allows the processing of large data cubes to be split across multiple computing nodes. As a result of these efforts, sofia 2 is substantially faster and comes with a much reduced memory footprint compared to its predecessor, thus allowing the large WALLABY data volumes of hundreds of gigabytes of imaging data per epoch to be processed in real time. The source code has been made publicly available to the entire community under an open-source licence. Performance tests using mock galaxies injected into genuine ASKAP data suggest that in the absence of significant imaging artefacts sofia 2 is capable of achieving near-100 per cent completeness and reliability above an integrated signal-to-noise ratio (SNR) of about 5–6. We also demonstrate that sofia 2 generally recovers the location, integrated flux, and w20 line width of galaxies with high accuracy. Other parameters, including the peak flux density and w50 line width, are more strongly biased due to the influence of the noise on the measurement. In addition, very faint galaxies below an integrated SNR of about 10 may get broken up into multiple components, thus requiring a strategy to identify fragmented sources and ensure that they do not affect the integrity of any scientific analysis based on the sofia 2 output.
The compact configuration of Phase II of the Murchison Widefield Array (MWA) consists of both a redundant subarray and pseudo-random baselines, offering unique opportunities to perform sky-model and ...redundant interferometric calibration. The highly redundant hexagonal cores give improved power spectrum sensitivity. In this paper, we present the analysis of nearly 40 hr of data targeting one of the MWA's epoch of reionization (EoR) fields observed in 2016. We use both improved analysis techniques presented in Barry et al. and several additional techniques developed for this work, including data quality control methods and interferometric calibration approaches. We show the EoR power spectrum limits at redshift 6.5, 6.8, and 7.1 based on our deep analysis on this 40 hr data set. These limits span a range in k-space of 0.18 h Mpc−1 < k < 1.6 h Mpc−1, with a lowest measurement of Δ2 ≤ 2.39 × 103 mK2 at k = 0.59 h Mpc−1 and z = 6.5.
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 × 1.28 MHz MWA images across the 170-200 MHz band taken at 0.5 s time resolution at the known dispersion measures (DMs) and arrival times of the bursts and searched both within the ASKAP error regions (typically ∼10′ × 10′), and beyond (4° × 4°). We identified no candidates exceeding a 5 threshold at these DMs in the dynamic spectra. These limits are inconsistent with the mean fluence scaling of = −1.8 0.3 ( , where is the observing frequency) that is reported for ASKAP events, most notably for the three high-fluence ( 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 turnover 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/104 K)−1.35 pc for FRB 171020.
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 realize 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, PSR J0437−4715 and PSR J2145−0750, made with these facilities; MWA observations sampled multiple frequencies across the 80-250 MHz frequency range, while the EDA provided direct-sampled baseband data to yield a large instantaneous usable bandwidth of ∼200 MHz. Using these exploratory 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.
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 a time resolution of 0.5 s and a frequency resolution of 10 kHz. 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 narrowband signals, (2) persistent narrowband signals, and (3) impulsive broadband signals. We found no such signals with nonterrestrial 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 experiments 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.
Abstract
Recently, low-frequency, broad-band radio emission has been observed accompanying bright meteors by the Long Wavelength Array (LWA). The broad-band spectra between 20 and 60 MHz were ...captured for several events, while the spectral index (dependence of flux density on frequency, with Sν ∝ να) was estimated to be −4 ± 1 during the peak of meteor afterglows. Here we present a survey of meteor emission and other transient events using the Murchison Wide Field Array (MWA) at 72–103 MHz. In our 322 h survey, down to a 5σ detection threshold of 3.5 Jy beam−1, no transient candidates were identified as intrinsic emission from meteors. We derived an upper limit of −3.7 (95 per cent confidence limit) on the spectral index in our frequency range. We also report detections of other transient events, such as 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.
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 dedispersion is essential, especially at frequencies below 300 MHz. We present the lowest-frequency (80-220 MHz), coherently dedispersed detections of one of the most promising pulsars for current and future PTAs, PSR J2241-5236, using our new beamformer software for the MWA's voltage capture system, which reconstructs the time series at a much higher time resolution of ∼1 s by resynthesizing the recorded voltage data at 10 kHz/100 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 determinations with very high precision, of the order of (2-6) × 10−6 , 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.
Rare intermittent pulsars pose some of the most challenging questions surrounding the pulsar emission mechanism, but typically have relatively minimal low-frequency ( 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 ongoing 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 parameterized by a log-normal distribution at both frequencies, contrary to previous results that suggested a power law distribution. Further low-frequency observations are required in order to explore in detail aspects such as pulse-to-pulse variability and intensity modulations, as well as 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 suggest 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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