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Principal Scientist update
EoR Science Overview
An End-to-End Simulation for SKA1-LOW Observation
The removal of strong foreground contamination is one of the primary challenges for detecting the redshifted 21cm signal from the Epoch of Reionization (EoR). An end-to-end simulation that mimics a realistic astrophysical foreground, as well as the observational and instrumental effects, is necessary to develop and justify various pipelines of detection of the 21cm signal from EoR. In this work, we present an end-to-end simulation of SKA1-LOW observation at three separate frequency bands at 60-70 MHz, 110-120MHz, and 160-170 MHz, respectively, which has a pixel size of 5", a frequency resolution of 20kHz and an FoV of 100 square degrees. For astrophysical foregrounds, we consider the Galactic synchrotron and free-free emission, extragalactic AGN, and radio halo from galaxy clusters. The SKA1-LOW configuration with antenna response is implemented using the OSKAR software, and we also consider the antenna's thermal noise, gain and phase error, and interference from the ionosphere. The simulation will be helpful for the SKA user community, especially for developing and verifying the algorithm that detects the EoR signal from the actual observation.
There are many ways in which systematics can manifest within a radio telescope signal chain. Some are easily understood and trivial to mitigate, while others can compromise data quality in subtle ways if they evade detection. An extensive ecosystem of tools has been built around the detection of various kinds of defects, each with its own strengths and flaws.
The MWA EoR group has developed their own approach to this problem, using Nextflow to apply novel statistical and signal processing techniques to MWA data at an unprecedented scale. Dev presents a taxonomy of systematics that this pipeline has uncovered, and an overview of the methods used to detect them.
GEG Science Overview
Refining the 200 MHz Local Radio Luminosity Function
We revisit prior work on the 200 MHz local radio luminosity function (RLF) to produce the most accurate measurement to date. We combine the low-frequency (70 − 230 MHz) Galactic and Extragalactic Murchison Widefield Array (GLEAM) survey with the 6dFGS spectroscopic survey. We build on recent work (Franzen et al., 2021b) by (a) including source previously excluded (‘A-Team’ radio sources and other bright sources missed, (b) using radio morphology and radio-far-infrared correlation to confirm, or in some cases, refine the radio source classification, (c) imposing an upper redshift limit of z = 0.1 and (d) using the Page & Carrera (2000) method for more accurately determining the RLF. This last choice allowed us to use Poisson uncertainties when fitting the RLF. The reassessment of the source classification led to 26 star-forming galaxies (SFGs) from F21 being reclassified as active galactic nuclei (AGN). The sample included 19 new sources including four A-Team sources, 13 new AGN and two bright local SFGs (NGC 254 and M 77). The morphological classification allowed us to identify samples bent, S-shaped and one-sided AGN. The overall RLFs for SFG and AGN are similar to before with slight changes at the bright and faint ends where small numbers sill hamper accurate measurements. From their WISE colours only a small fraction (∼ 15%) are found to have high (>0.01) Eddington accretion rates.
GLEAM-X: Where are we at and what's next?
The GaLactic and Extragalactic All-sky MWA eXtended(GLEAM-X) survey boasts twice the resolution and up to an order of magnitude higher sensitivity to its predecessor the GLEAM survey. In 2020, observations for GLEAM-X were completed and the first data release was published earlier this year covering 2,000 square degrees down to an RMS noise level of <1.5 mJy/beam. The team is now working on the second data release covering an RA range from ~20h to 6h. In this talk, we will present the status of the GLEAM-X survey, lessons learned on ionospheric behaviour and update timelines for the next early data release to collaboration.
Randall Wayth, Yajun Wu
Principal Engineer Update and SHAO Receiver Update
MWA Archive Update
In this presentation, I will provide an update on the MWA's data archive, MWA ASVO and the implementation of the data retention policy.
Fringe-stopping for the MWA/MWAX
Fringe stopping prior to cross-correlation allows longer visibility integration times without introducing de-correlation, which in turn allows the size of the correlator output products to be substantially reduced. The new correlator for the MWA, "MWAX", was designed from the outset to include pre-correlation fringe stopping. However, it was not supported in the first operational release and users were required to perform phase corrections post-correlation within tools such as Birli or pyuvdata. Pre-correlation fringe stopping has now been fully integrated into MWAX and validation testing is in progress. Subject to final sign-off, the intention is for fringe stopping to be made available as an optional user setting on all observations taken from the beginning of 2023 onwards. This talk will describe MWAX's implementation of fringe stopping and present validation test results to date.
Devojyoti Kansabanik, Divya Oberoi
Solar Science with the MWA : Status and Plans
It has been an exciting year for the solar science group of the MWA. We have had significant developments on both fronts - developing novel calibration and imaging alogrithms and pipelines, as well as using them to pursure novel science. On the techniques front the sperctro-polarimetric snapshot images from our pipeline now defines the state-of-the-art and achieve polarisation purity on par with quality non-solar radio images. On the science front our explorations have ranged from the weakest non-thermal emissions ever detected from the quiet Sun to the strong radio emissions associated with the drivers of space weather and are yielding very interesting insights and discoveries. I will present an overview of the different aspects of work we have been pursuing over the past some time along with our plans for the near future.
Detecting and characterising weak radio emissions from the Quiet Solar Corona
Observations spanning across wavelengths ranging from X-rays, Extreme Ultraviolet, optical and radio bands have demonstrated that copious amounts of nonthermal particles are generated during solar flares. Numerical simulations have also become more and more realistic and for the first time produced nonthermal particle spectra consistent with observations. However most observations of these nonthermal particles are focussed on active regions, particularly on big flares, where huge amounts of nonthermal particles are produced. While simulations can now qualitatively explain these properties, they also predict a regime of magnetic reconnection where particle acceleration would be very weak, leading to very small nonthermal signatures. To test our understanding of particle acceleration, it is critical to detect and characterise the nonthermal emissions from weak flares. Here we focus on detecting these signatures using the radio band. The first work of this nature at low radio frequencies, with focus on the quiet solar corona, was done by Mondal et al. (2020), where they detected ubiquitous radio emissions from the quiet solar corona. They postulated that these are the nonthermal signatures of the long hypothesised “nanoflares” and are evidence that not only are nanoflares happening in the quiet solar corona, but are also accelerating particles to nonthermal energies. Due to its importance, this work has been followed up on several fonts, ranging from investigating the validity of these results in independent datasets, using machine learning tools to characterise them, studying the spectral nature of these transient emissions and also studying them at higher frequencies. In this presentation, I will give a brief summary of the various works we have done for detecting and understanding the nonthermal nature of the quiet solar corona.
First detailed polarimetric study of a group of type-III solar radio bursts with the Murchison Widefield Array
Magnetic reconnection is a well-known process for the acceleration of electrons in the solar corona. When streams of semi-relativistic electrons travel through the hot magnetized coronal plasma along open magnetic field lines, it can result in type-III solar radio bursts by plasma emission mechanism. These radio emissions are among the most widely studied solar phenomenon at meter-wavelength. However, most of these studies are limited to the analysis of their dynamic spectra, which do not provide any information about the structure and location of these sources. The emergence of new generation telescopes like the Murchison Widefield Array, and our recently developed full Stokes calibration and imaging pipeline (P-AIRCARS, Kansabanik et al. 2022), now enables us to produce spectro-polarimetric snapshot solar radio images with high fidelity and dynamic range. This allows us to obtain polarimetric properties of these bursts in unprecedented detail and has already led to a few interesting discoveries. Circular polarization of type-III radio bursts are well reported in the literature. Theoretically one expects any linear polarization, even if present, should get washed out due to the large differential Faraday rotation in the corona. On the contrary, we have found the first convincing image-based evidence of linearly polarized emission from these bursts. We note that the linear polarization fraction is greater than the circular polarization fraction. Here we will present our findings about the polarization properties of these type-III solar radio bursts.
First detailed polarimetric study of a type II solar radio burst with the Murchison Widefield Array
Type II solar radio bursts are well known to be predominantly associated with the more energetic and fast coronal mass ejections (CMEs). These CMEs are expected to drive shocks in the coronal medium and play a dominant role in giving rise to energetic particles which are the biggest concern for Space Weather. Type II radio bursts arise from plasma emission mechanisms and occur at fundamental and harmonic levels of the local plasma frequency. The emission at both the fundamental and harmonic is often found to be split into two sub-bands or lanes. A commonly accepted interpretation is that the electrons accelerated at the shock front moving both ahead of and behind the shock, giving rise to these two lanes of emission. This suggests that these sources must lie close to each other. The vast majority of studies of type II bursts, however, rely on dynamic spectra which do not provide any spatial information. High fidelity and dynamic range solar radio images with good temporal and spectral resolution from instruments like the Murchison Widefield Array (MWA) now enable such imaging studies. Interestingly, a recent MWA imaging study of the harmonic emission from a type II burst (Bhunia et al., 2022 A&A, submitted) finds evidence that the sources from the two lanes are not only located rather far apart in the radio images, but they are also moving in different directions and with different speeds. Making use of the recently developed spectro-polarimetric snapshot imaging pipeline (P-AIRCARS, Kansabanik et al., 2022), we have extended the study of Bhunia et al. 2022 to include polarimetric imaging and also improved upon some of the analysis procedures used by them. Here we summarize the preliminary results of the polarization properties of this type-II radio burst.
Recent developments in space weather research with spectro-polarimetric imaging using MWA
Low-frequency radio observations have been expected to serve as a powerful tool for Space Weather observations for decades. This is because (1) radio observations are sensitive to a wide range of Space Weather-related observations ranging from emissions from coronal mass ejections (CMEs) to studies of the solar wind; and (2) the ground-based radio observatories provide high sensitivity data at high time and spectral resolution, which remains a challenge for most space-based observatories. While radio techniques like Interplanetary Scintillation (IPS) are well established in Space Weather research, radio imaging studies have remained technically challenging. This is now changing with the confluence of data from the Murchison Widefield Array (MWA), and the robust unsupervised analysis pipelines developed by our group. Recently, we have implemented a precise polarization calibration pipeline, which delivers full Stokes radio images with unprecedented fidelity and dynamic range. This tool is now enabling the potential of low-frequency radio observations for heliospheric and Space Weather research. An example includes measuring plasma parameters and magnetic fields of CMEs out to 8.5 solar radii using gyrosynchrotron modeling of full Stokes spectra. We will also share the current status of the objective to measure the heliospheric Faraday rotation towards numerous background linearly polarized radio sources with the Sun in the field of view.
Identification of a heliospheric transient using Interplanetary Scintillation observations with the Murchison Widefield Array
Our current understanding of coronal mass ejections, as well as other solar events in interplanetary space, is well-developed to forecast the effects on Earth. However, it is still insufficient in its ability to predict the evolution of these events with a high degree of accuracy. Considering that a major space weather event could put crucial technologies at risk, predicting the severity of such events is of great importance to the space weather community. Interplanetary scintillation (IPS) is a phenomenon which causes sufficiently small radio sources to “twinkle” in the solar wind. Since its discovery in the mid-1960s, it has been developed as a powerful astrophysical tool, both for studying compact (<1 arcsecond size) objects, and for studying the solar wind. In the last few years, we have breathed new life into this old technique, adapting it for modern low-frequency instruments such as the Murchison Widefield Array (MWA). The key advance we have made is to exploit the enormous field of view of the MWA. This allows us to monitor all IPS sources across a field of view 30 degrees across, leading to an unprecedented density of measurements. A recently completed survey of IPS sources in the sky above the MWA has become the basis of this work. We have conducted a blind search of 49 days of MWA IPS observations from mid-2019, with overlapping daily observations approximately East and South-East of the Sun at an elongation of 30 degrees. This search has revealed several interesting transient features characterised by higher than usual scintillation levels (in spite of the observations being taken at solar minimum). None have (yet) been linked to any known solar events. However, one solar wind enhancement is captured in two observations several hours apart, allowing the plane-of-sky velocity to be inferred.
KEYNOTE, DAY 1
Novel insights and discoveries pertaining to weak solar activity with MWA
Weak solar radio bursts are associated with ubiquitous heating and particle acceleration activity in the solar corona. Since metrewave frequencies are sensitive to emission across coronal heights as a function of frequency, spectroscopic snapshot imaging facilitates tomographic exploration these active phenomena. The advent of simultaneous wideband high dynamic range imaging with MWA enabled the study of bust sites using spatially resolved dynamic spectra at sub-s and sub-MHz resolution. I will summarize some of the novel discoveries, insights and research avenues these studies have unveiled. This will include the discovery of ~s timescale quasi-periodic pulsations in burst source structure in tandem with its intensity - their properties and implications for local physical fields. I will also present our exploration of coronal turbulence in the inner corona.
Calibrating the MWA as a dual-polarised instrument is notoriously difficult, evidenced by the fact that even our latest beam models and calibration algorithms produce solutions that demonstrably contain frequency-dependent phase errors between the two polarisations. In this talk, I will present an overview of the basic problem using a combination of intuitive and mathematical arguments, and briefly outline some of the approaches (known to me) that different groups have implemented to overcome it, including some musings on the possible role that pulsars could play in the solution. I will argue that despite the heroic efforts of multiple groups, we still lack a fundamental understanding of how the remaining phase errors are creeping into our calibration solutions in the first place. I will conclude by advocating for a more centralised approach to solving the problem, as well as to making known solutions and workarounds more accessible to the broader MWA community.
A Near-Field treatment of Aperture Synthesis techniques using the Murchison Widefield Array
Typical radio-interferometer observations are performed assuming the source of radiation to be in the far-field of the instrument, resulting in a Fourier relationship between the observed visibilities in the aperture plane and the sky brightness distribution. When such observations are performed of events in the near-field, radiation with curved wavefronts are correlated with far-field delays resulting in loss of signal coherence in the reconstructed images of the event. In this talk, I will briefly discuss the near-field aperture synthesis techniques developed using the Murchison Widefield Array, using a single Phase 3 observation of the ISS (as it appears as a bright near-field event). Much like how the phase center of an observation can be changed post-observation, we can perform visibility phase corrections to bring the observed near-field event into the ‘focus’ of the array. Using animations of the MWA ISS observation, I show how the visibility sampling in the aperture plane (UVW grid) curves to match the curvature of the near-field wavefront, enabling coherent image reconstruction of the near-field signal. Finally, I conclude my talk by demonstrating how the distance to the near-field events can be inferred from the phases of the visibilities, i.e, inverting the curvature of the radiation to obtain a range measurement.
A commensal MWA beamformer for SETI and FRBs
The MWA, like many radio array telecopes, uses Ethernet packets to route data from the receiver 'frontend' to the MWAX correlator 'backend'. Using a routing feature called Multicast, these data packets can be sent to any backends that request a copy of the datastream. Here, we report on the commensal MWA beamformer system: a collaboration between ICRAR researchers and the Breakthrough Listen search for life beyond Earth. The commensal system is being used to undertake a technosignature search (SETI, the Search for Extraterrestrial Intelligence), and Fast Radio Bursts (FRBs). We will give an overview of how the system works, current status, and future plans.
KEYNOTE, DAY 2
Long period radio transients
One of the most exciting discoveries in MWA data has been an unusual periodic radio transient, repeating every 18.18 minutes, with the discovery published inNatureearlier this year. The source's long period makes it challenging to explain the radio emission with conventional theories of pulsar emission, yet the high polarisation and pulse morphologies are similar to what would be expected from magnetic neutron stars. The high radio brightness and short activity window of the first discovery challenges us to look for fainter examples, in the hopes of elucidating the properties of the full population. This year, we ran a monitoring campaign of the Galactic Plane, with the explicit goal of detecting new transients. The campaign was highly successful, and in this talk I will outline some of the new discoveries, and what this tells us about this intriguing new class of sources.
SMART Animation, Transients and PFT Science Overview
Planning the next stage of the SMART pulsar survey
The Southern-sky MWA Rapid Two-metre (SMART) pulsar survey is an ambitious project, with prospects of discovering hundreds of new pulsars, and will be a valuable prototype for future SKA-Low pulsar surveys. Now that the first ("shallow") stage processing of SMART data is nearing completion, we have started planning for the full survey processing workflow in earnest. I will present our current plans and developments for the second ("deep") stage processing, which are informed by lessons learned from the first stage. Our goal is to start the second-stage processing of SMART data by April 2023. Pulsars abound!
A targeted search for repeating fast radio bursts with the MWA
In this talk, I will present a targeted search for low-frequency (144--215MHz) FRB emission from five repeating FRBs using 23.3hr of archival data taken with the Murchison Widefield Array (MWA) Voltage Capture System (VCS) between 2014 September and 2020 May. This is the first time that the MWA VCS has been used to search for FRB signals from known repeaters, which enables much more sensitive FRB searches than previously performed with the standard MWA correlator mode. We performed a standard single pulse search with a temporal and spectral resolution of 400us and 10kHz, respectively, over a 100pc cm^-3 dispersion measure (DM) range centred at the known DM of each studied repeating FRB. No FRBs exceeding a 6sigma threshold were detected. The fluence upper limits in the range of 32-1175Jy ms and 36-488Jy ms derived from 10 observations of FRB 190711A and four observations of FRB 201124A respectively, allow us to constrain the spectral indices of their bursts to >-1 if these two repeaters were active during the MWA observations. If free-free absorption is responsible for our non-detection, we can constrain the size of the absorbing medium in terms of the electron temperature T to <1.00(T/10^4K)^-1.35pc, <0.92(T/10^4K)^-1.35pc and <[0.22--2.50]*(T/10^4K)^-1.35pc for FRB 190117A, 190711A, and 201124A, respectively. However, given that the activities of these repeaters are not well characterised, our non-detections could also suggest they were inactive during the MWA observations.
The curious case of subpulse drifting and nulling in pulsar J0026-1955
Pulsars are excellent astrophysical laboratories for studying physics under extreme conditions such as ultra-strong gravitational and magnetic fields. Admittedly, after five decades since their discovery, physical processes governing their emission mechanism remain poorly understood. PSR J0026-1955, which was independently discovered by the MWA (McSweeney et al. 2022), exhibits unusual sub-pulse drifting characteristics, a large (~70%) nulling fraction and mode changing. These properties make it an excellent addition to a small subset of promising targets for uncovering the intricacies of the pulsar emission mechanism. I will present analysis and results from follow-up observations of this pulsar made with the upgraded Giant Meterwave Radio Telescope (uGMRT) spanning the frequency range 300-750 MHz. Our analysis confirms quite a peculiar sub-pulse drifting behaviour seen in MWA data, including at least two distinct drifting modes, rapid changes between the modes and an evolution of drift rate within a mode. Further, our analysis also reveals the evolution toward a faster drift rate is usually followed by a null sequence, and there is also some evidence for memory across nulls. With all these intriguing properties, PSR J0026-1955 makes an ideal testbed for testing the carousel model of sparks and holds the potential to uncover the intricacies of pulsar emission physics.
Image-based searches for pulsar candidates using MWA VCS data
Pulsars have been studied extensively over the last few decades and have proven instrumental in exploring a wide variety of physics. Discovering more pulsars emitting at low radio frequencies is crucial to further our understanding of spectral properties and emission mechanisms. The Murchison Widefield Array Voltage Capture System (MWA VCS) has been routinely used to study pulsars at low frequencies and discover new pulsars. The MWA VCS offers the unique opportunity of recording complex voltages from all individual antennas (tiles), which can be off-line beamformed or correlated/imaged at millisecond time resolution. Devising imaged-based methods for finding pulsar candidates, which can be verified in beamformed data, can accelerate the complete process and lead to more pulsar detections. Image-based searches for pulsar candidates can reduce the number of tied-array beams required, increasing compute resource efficiency. Despite a factor of ~ 4 loss in sensitivity, searching for pulsar candidates in images from the MWA VCS, we can explore a larger parameter space, potentially leading to discoveries of pulsars missed by high-frequency surveys such as steep spectrum pulsars, exotic binary systems, or pulsars obscured in high-time resolution timeseries data by propagation effects. Image-based searches are also essential to probing parts of parameter space inaccessible to traditional beamformed searches with the MWA (e.g. at high dispersion measures). In this talk I will describe the innovative approach and capability of dual-processing MWA VCS data, that is forming 1-second visibilities and sky images, finding pulsar candidates in these images, and verifying by forming tied-array beam. We developed and tested image-based methods of finding pulsar candidates, which are based on pulsar properties such as steep spectral index, polarisation and variability. The efficiency of these methodologies has been verified on known pulsars, and the main limitations explained in terms of sensitivity and low-frequency spectral turnover of some pulsars. No candidates were confirmed to be a new pulsar, but this new capability will now be applied to a larger subset of observations to accelerate pulsar discoveries with the MWA and potentially speed up future searches with the SKA-Low.
There were 95 registered participants for the meeting.
Mia Walker/Aoife Stapleton
Ash Nambiar & Venus Chico
Best talk award: "Pathological Systematics and Where to Find Them" by Dev Null, judged by Principal Scientist Chris Riseley.
Registration/draw prize: Jun Tian
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