We request MWA follow-up of neutrino transient candidates detected by the ANTARES and IceCube neutrino telescopes. These follow-up observations will either lead to the strongest limits to date on prompt radio emission from neutrino transients, or to the detection of a correlated radio signal. The latter will lead to the localization of the astrophysical sources that emit high-energy neutrinos. In addition, the combined particle and radio detection from a common astrophysical source will contribute to evidence for the origin of cosmic rays. For around 30% of the ANTARES and IceCube neutrino alerts, the source is expected to be visible immediately from the MWA site. We expect to receive and follow up three or four neutrino triggers during 2020-B. The follow-up observations would be disruptive target of opportunity observations. We request 30 minutes of prompt follow-up of each trigger, followed by a second epoch 1–2 weeks later for each trigger (matched in LST range) for comparison.
Follow-up to the Long Baseline Epoch of Reionisation Survey
One of the principal systematic constraints on the Epoch of Reionisation (EoR) experiment using the Murchison Widefield Array (MWA) is the accuracy and depth of the foreground calibration model. Given the large field of view of the MWA, the MWA EoR fields contain several bright, extended sources located towards the edge of the primary beam and in the sidelobes. Recent results have shown that accurately modelling and removing bright sources at the edges of the field and in the sidelobes, removes more contaminating signal than just removing sources at the centre of the primary beam. In an effort to improve the models of sources in the MWA primary beam sidelobes of the EoR0 and EoR1 fields, we are conducting the Long Baseline Epoch of Reionisation Survey (LoBES). This survey consists of multi-frequency (between 103 --230 MHz) observations of EoR0, EoR1, and their eight neighbouring fields using the MWA Phase II extended array. Here we proposeto re-observe key LoBES fields to replace bad data taken during the original LoBE survey.
MWA Observations of the Sun
Two hundred hours of observing time for solar observations is requested during the 2020-B observing semester. These data will be used to address science objectives for solar burst science(Goal A); studies of weak non-thermal radiation (Goal B); quiet sun science (Goal C), imaging of CME plasma (Goal D); and observations coordinated with various spacecraft (Goal E). Goal A will focus on detailed investigations of individual events seen in the MWA data, using the unsurpassed spectroscopic snapshot imaging ability of the MWA to address some key solar physics questions. Detailed observations of type II bursts and type III bursts will be one focus. Goal B will address studies of the numerous short lived and narrow band emission features, significantly weaker than those seen by most other instruments revealed by the MWA. These emission features do not resemble any known types of solar bursts, but are possible radio signatures of the “nanoflares' which have long been suspected to play a role in coronal heating. A large database of these events is needed to be able explore this possibility and to reliably estimate their contribution to coronal heating. These observations will contribute to this database. Goal C will focus on characterising the Sun’s background thermal emissions, their short and long term variability and look for evidence of a scattering disc around the Sun. Goal D makes use of our high dynamic range capability to directly image the gyrosynchrotron emission from the CME plasma. Goal E proposes opportunistic coordinated observations with the Parker Solar Probe, the Solar Orbiter and the Chandrayaan-2.
High Time Resolution Observations of the Sun using the Voltage Capture System
The MWA Solar physics group has recently reported the discovery of ubiquitous weak impulsive nonthermal emissions from the quiet Sun. So far, they satisfy all of the requirements for being relevant for heating of the quiet corona. The median duration of these “events” was found to be close to the time resolution of the interferometric data, and most of these events remain unresolved intime. The objective of this proposal is to seek a small amount of VCS data (1 hour) to carry out high time resolution observations of these events. The present times of low solar activity and the MWA being in the extended configuration are particularly well suited for these observations. An hour of correlator observations for calibration is also requested.
Rapid-response MWA observations of Swift and Fermi gamma-ray bursts
We request the use of the MWA rapid-response mode to perform triggered standard correlator and VCS observations of Swift and Fermi gamma-ray bursts (GRBs) during the 2020B semester. The prompt and early-time radio emission associated with GRBs is still a poorly explored regime, particularly at MHz frequencies. Short-duration GRBs (SGRBs), one of the two main classes of GRBs, are a hot topic in astronomy as they are linked with the compact binary coalescence of binary neutron stars (BNS), or a neutron star (NS) - black hole (BH) binary. BNS mergers are the main classes of gravitational wave events known to have electromagnetic counterparts (Abbott et al. 2017). Several theories predict such mergers should produce prompt, coherent emission (such as fast radio bursts, FRBs; Totani, 2013; Falcke & Rezzolla, 2014; Zhang, 2014) the detection of which would allow us to distinguish between different binary merger models and scenarios. It is also possible that low-frequency pulsed radio emission could be generated by long-duration GRBs (LGRB; the other main GRB class resulting from stellar collapse; Usov & Katz, 2000).As prompt radio emission becomes delayed with decreasing frequency due to dispersion, such signals associated with GRBs may not arrive for seconds up to several minutes following the initial burst alerts at MWA frequencies. Given that the MWA rapid-response mode can automatically repoint the telescope within 14 seconds of receiving an alert, MWA is uniquely capable of being on-target in time to observe the earliest prompt emission. An additional advantage of the MWA is its large field-of-view, making it possible to follow-up Fermi detected GRB events, which have poor position constraints (order of ~10deg). Such rapid-response MWA observations have the sensitivities necessary to rule out some GRB models, which will in-turn constrain different neutron star equation-of-state models. These experiments also directly test transient strategies for SKA-Low.
Detecting ionised bubbles in the early Universe with stacked observations around Lyman-Alpha Emitting Galaxies
Measuring the evolution of the ionisation state of the intergalactic medium provides key insights into the growth of structure in the early Universe. Multiple observational probes have constrained the neutral fraction at a given redshift (e.g., sightlines to distant quasars, quasar proximity zones)or as an integrated quantity (e.g., Thomson scattering of CMB photons), yielding limits on the duration and endpoint of reionisation. The Silverrush Survey using the Subaru Hypersuprime Cam detected 282 bright Lyman-α Emitting Galaxies (LAEs) at z = 5.7, 6.6 in the UD-SXDS field (RA=34deg, Dec=-5deg). Strong Ly-αemission at the tail end of reionisation is suggested to be evidence for the host galaxy residing inside ionised regions of the IGM. Theoretical and simulation studies show an anti-correlation of21cm emission and Ly-α emission on scales of 5-50 cMpc (2-10’), as the 21cm traces neutral IGM while the Ly-α traces ionised bubbles. The expected brightness temperature contrast between neutral and ionised regions at z = 6.6 is2-10 mK, depending on the hydrogen neutral fraction. This is inaccessible without hundreds of hours of observations with the MWA for a single LAE. However, stacking 21cm images at known LAE spatial and redshift locations improves the signal-to-noise ratio leading to a statistical constraint on the IGM neutral fraction. Combined compact and extended array MWA observations of the UD-SXDS field at redshifted 21cm emission in these bands will produce images with sufficient angular resolution and surface brightness sensitivity to perform this stacking experiment. We have previously obtained 20.5 hours of data with the Compact configuration and propose to observe the same field with 40 hours of LST-matched observations with the Extended Array to complete the dataset. An IGM neutral fraction exceeding 25% at z = 6.6, should be detectable with SNR ≅ 3 using the 58 catalogued LAEs in the field.
GaLactic and Extragalactic All-Sky MWA-eXtended (GLEAM-X) survey
We propose the second half of observations of an extension to the highly successful GaLactic and Extragalactic MWA (GLEAM) survey. The aim is to create a legacy data set for innovative low-frequency science which will serve many astronomers overcoming years. This proposal covers an RA range of 20 to 06h, concluding the observation program initiated in 2018-A. The optimal time for our observations is a month between mid-September and mid-November, which straddles 2020-A and B, so we are submitting this proposal now for ease of scheduling. A deeper all-sky survey at higher resolution will enable a legion of science capabilities, whilst maintaining advantages over LOFAR including larger field-of-view (and survey speed), wider frequency coverage, and better sensitivity to extended emission. We propose to cover the GLEAM frequency range of 72—231 MHz and use repeated drift scans to observe 8,000 deg2, which in combination with the 20,000 deg2 observed in2018-A, will cover the entire southern sky. We will continue the successful snapshot imaging and image-plane combination strategy of GLEAM. We will utilise calibration strategies we have developed over the last year of research. The GLEAM extragalactic sky catalogue improves the prospects for good ionospheric calibration in this new regime, as well as trivialising absolute flux density calibration. Extrapolating from GLEAM, GLEAM-X will have lower noise, higher surface brightness sensitivity, and considerably wider bandwidth than TGSS. These properties will enable a wide range of science, such as:
Detecting and characterising cluster relics and haloes beyond z = 0.45;
Measuring the low-frequency luminosity function to z~0.5 (particularly the bright end);
Characterising the low-frequency polarised sky;
Investigating the spectral energy distributions (SEDs) of selected AGN to characterise jet activity;
Providing broad-band radio SEDs of up to 1 million radio sources (especially in combination with the Rapid ASKAP Continuum Survey (RACS));
Better constraining the typical ionospheric diffractive scale and weather at the MRO, feeding into SKA_Low calibration strategies.
In addition, the proposal is designed to be commensally used for transients science.
Timing and imaging follow-up of the first pulsar discovery from the MWA
We request 4 hr of MWA time for continued timing observations of PSR J0036-1033 – the first pulsar discovery from the MWA. The pulsar was discovered in the initial processing of data from the ongoing SMART pulsar survey (G0057), where ~3% of the data were searched out to DMs ~300pc cm3,reaching 30% of the full array sensitivity. Thanks to the virtues of the MWA’s large FoV and the voltage capture mode of our survey, we have been able to perform a number of immediate follow-up investigations, including the processing of archival data for candidate confirmation, initial positional determination, and a first attempt on polarimetry. The pulsar appears to be relatively faint, with inferred flux densities ~2-8 mJy at 150 MHz and an implied luminosity ( 0.1-0.3mJy kpc2at 1400 MHz)that would place it near the top end of the so-called low-luminosity pulsars, of which only a few are known. We have already commenced its timing observations (via the DDT proposal D0029), and the primary goal of this proposal is the continuation of that effort, in order to determine the pulsar’s spin and astrometric parameters. A secondary goal is to perform improved polarimetry, in an attempt to better constrain the pulsar’s emission geometry. The proposal will also complement the planned follow-up efforts with Parkes that will help constrain the pulsar’s spectral index and luminosity, alongside allowing us to study the spectral evolution of its emission properties.
Demonstrating advanced space domain awareness capability with the MWA
Over the last 7 years we have shown that the MWA can be used to detect and monitor satellites and space debris in low Earth orbit. The initial detection of the international space station (ISS, Tingay et al. 2013) used correlated observations and image-based searches to track the ISS as it transited the MWA in elevation and azimuth. Subsequent work was done with voltages to perform traditional radar processing (Palmer et al. 2017, Hennessey et al. 2019) to also recover line of sight range, velocity, and acceleration. Since these two demonstrations, we have used the MWA as a passive bi-static radar receiver station to detect aircraft, meteor trails, satellites, and space debris (Zhang et al. 2018). Our most recent work (Prabu et al. 2020, Prabu et al. 2021 submitted) focuses on the development of an image-based processing workflow that automates the detection and monitoring of resident space objects in Low Earth Orbit (LEO). Work to date has focused on demonstration of capability using archival observations from the MWA. In this proposal we are requesting a modest 8h12m of observations to test and develop an advanced capability of the MWA using observing modes that are not present in the archive (specifically the choice of frequency channels and low elevations). The proposed observations will coincide with activities leading up to and including the Space Fest event that is a civilian activity run by the Australian Airforce at the end of November, and the Hayabusa-2 capsule re-entry in early December.
Triggering on ASKAP FRB detections
Since the early discoveries by Lorimer et al. (2007) and Thornton et al. (2013), the field of Fast Radio Bursts (FRBs) has been through a number of significant developments: most notably, interferometric localisations of several, and an explosion in their tally with the CHIME/FRB project coming online. Measurements of their scattering, scintillation, polarisation and Faraday rotation properties are becoming routine, and theoretical efforts to understand their origin continues unabated. With its large field of view( 30deg2) and interferometric advantages, the Australian SKA Pathfinder (ASKAP) has proven to be a uniquely capable instrument for detecting them, and localising them with arcsecond (or better) precision. The MWA’s co-location at the MRO site provides unique opportunities for undertaking efficient co-observing and triggering observations. Our shadowing campaign in early years (2018-2019) has placed the most stringent constraints on the low-frequency emission of FRBs and their spectral indices. Starting the 2020A semester, we have shifted the focus to further development and testing of VCS triggering strategies and optimising them for improved efficiency. Simultaneous detection of even a single FRB would mean a huge payoff and will yield the first unambiguous constraints on the spectral and scattering properties of FRBs, besides putting an end to the long-unresolved puzzle relating to the lack of FRB emission at low frequencies. A detection within the MWA band (<300 MHz) will also help exclude certain classes of progenitor models that involve dense plasma surrounding FRB hosts, and thus narrow down on the plausible models for their origin.
Time domain science – High time resolution
Tracking Interstellar Space Weather Toward Timing-array Millisecond Pulsars
Searching for nanohertz gravitational waves (GWs) using a celestial array of extremely stable millisecond pulsars (MSPs) is amongst the high-profile science goals of the SKA and its pathfinders. The success of these Pulsar Timing Array (PTA) experiments will extend the spectrum of gravitational-wave astronomy that has been opened by the LIGO/VIRGO detections of kilohertz-frequency gravitational waves produced by black-hole and neutron-star mergers. PTAs exploit the clock-like stability of MSPs to make a direct detection of ultra-low frequency (nano-Hertz) gravitational waves, and the science is highly complementary to that possible with LIGO-like detectors. PTAs are most sensitive to GW signals produced by supermassive black-hole mergers. Interstellar propagation effects on pulsar signals are a major contributor to timing noise, which, if not accurately measured and corrected for in timing measurements, may ultimately limit the detection sensitivity of PTAs. The interstellar medium (ISM)effects are much stronger at low frequencies, and hence the MWA presents an exciting and unique opportunity to calibrate interstellar propagation delays. Here we propose continued regular observations of three promising PTA pulsars for which the MWA’s sensitivity and capabilities allow detailed studies. The primary goals include characterising the nature of the turbulent ISM through high-quality scintillation studies and investigating the chromatic (frequency-dependent) dispersion measures (DMs) via high precision DM determinations that are now possible with the MWA. Our success and accomplishments in this area will also help define the scope of a more ambitious program in the future, as upgrade plans move forward for the MWA and with the prospects of a pulsar monitoring capability with Phase3 MWA. This will also prove to be an excellent scientific niche for the MWA, and eventually for the SKA-low.
High-time resolution science – Pulsars
Detecting cosmic-ray extensive air showers
The goal of this project is to detect cosmic rays interacting in the atmosphere above the MWA. Using time granted in the previous allocation, we:
detected 7 cosmic ray candidates and are able to differentiate these from standard RFI events;
have streamlined the processing pipeline to increase the rate at which data is processed;
have demonstrated that the data can be processed and deleted within the necessary timeline.
Our goals for this proposal are to:
gather more data to increase our sample of cosmic ray candidates and further characterise these events;
continue to investigate the RFI on site at nanosecond timescales.
The verification of the first cosmic ray event at the MWA opens up opportunities in the development of cosmic ray reconstruction and analysis techniques and benefits transient science at nanosecond timescales. There have been two Australian Research Council grants (ARC Discovery Project DP200102643, Detecting cosmic rays using precision radio imaging; and ARC LIEF grant LE200100078, A particle detector array forth Murchison Widefield Array) awarded towards cosmic ray science at the MWA with the specific purpose of detecting and analysing many cosmic ray events, where this work will allow for a better understanding of the view that the MWA has of cosmic ray air showers.
Transients (Cosmic Rays)
An Interplanetary Scintillation Survey with the extended Phase II MWA.
In 2019A and 2020A we proposed (and were awarded) daily observations of Interplanetary Scintillation (IPS). We can use these observations for both space weather monitoring, and for determining the arcsecond-scale structure of the detected sources. With 8⨉10 minute observations per day we covered all solar elongations where we can make optimum IPS measurements, broadly following the survey methodology used in 2019A. We propose to continue these observations in 2020B with 10✕10 minute observations per day, since with the Sun in the Southern Hemisphere over 2020B we will be able to cover areas further to the North of the Sun while observing at a reasonable elevation. Overall, this semester will double our coverage. Observing later in the year than was possible in 2019/20A will allow us to access parts of the sky for which we have no prior data. Much of this area is particularly interesting as they cover high Galactic latitude fields well studied by other radio surveys such as FIRST. However, we will also be able to make IPS measurements in the Galactic Plane near the Galactic Centre for the first time with the MWA. Our observations in 2019A of the Galactic anticentre have demonstrated the unique ability of the MWA to probe scattering due to the ISM along a very large number of lines of sight. With an additional measurement above our standard observing frequency of 162MHz, we will be able to separate scatter broadening from source structure, and so we propose to observe some Galactic centre fields at the top GLEAM band. In summary, we request 190 hours to continue the survey started in 2019A and 2020A, and a further 20 hours to observe ~10 fields ~20 times at 200-230MHz.
SHI / GEG
Monitoring of X-ray binary transient outbursts with the MWA
We propose pointed observations with the MWA of any nearby bright (≥ 50 mJy) outbursting X-ray binary (XRB) during the 2020-B observing semester. The low-frequency regime of radio jets in XRBs is still not fully explored, especially at frequencies < 500 MHz. XRBs can produce two different types of radio jets over the course of a single outburst. Optically thick, at-spectrum, compact steady jets are observed during the hard state whereas steep-spectrum, relativistically-moving transient jets are detected near the peak of the outburst, when the source undergoes a transition from the hard to the soft X-ray spectral state. Both types of jets can be observed in the low-frequency regime. We aim to study both types of radio jets using MWA observations to constrain the radio spectrum in the low-frequency band. The prime aim of this proposal is to detect the possible low-frequency turn-over in the compact jets. The turn-over frequency along with the spectral slope below the turn-over frequency will put observational constraints on the electron energy distribution and magnetic field strength. Simultaneous high-cadence monitoring of transient jets at low and high frequencies will provide observational measurements essential for constraining the low-frequency emission geometry and thereby constrain various theoretical jet models. To the end, we aim to provide high-quality low- frequency radio light curves of XRBs covering a few epochs in the hard state, and denser sampling over the hard-to-soft state transition.
An MWA Probe of Radio Bridges in Merging Galaxy Cluster Pairs in the Low- Frequency Radio Band
Mpc-scale radio halos and radio relics have been frequently observed in galaxy clusters with merger signatures, showing a higher prevalence in luminous systems (see van Weeren et al. 2019, for a recent review). On the contrary, radio bridges, which appear as a giant filamentary source of diffuse radio emission that links two merging clusters, are rarely seen, possibly due to their low luminosities in the radio band. As of today, only two cases, i.e., the radio bridges in Abell 399 - Abell 401 (Govoni et al. 2019) and Abell 1758 (Botteon et al. 2020) have been detected by LOFAR with a threshold of 3σ at about 140 MHz, and both the bridges show a tight spatial correlation with a hot ( ~ 7 keV) and dense X-ray emitting region between the merging members. A possible explanation for the formation of radio bridges is that a population of in situ relativistic electrons are re-accelerated by either shock or turbulence. However, the form of the radio spectra of the radio bridge, the origin of the relativistic electrons, and the strength of the magnetic field, as well as the details of the electron acceleration processes are still unclear, due to the lack of observational evidence. Therefore, we propose to utilize the advantages of the MWA, i.e., the wide field of view, the good coverage of the low frequency band and the intermediate spatial resolution, to carry out deep pointing observations of two pairs of galaxy clusters (Abell 3016 - Abell 3017 and Abell 222 - Abell 223) across full MWA Phase II band 72-231 MHz for a total of 170 hrs (17 hrs per frequency band per source). Each of the two pairs of contain massive clusters in a clear dynamically disturbed state that are evolving and are seen in a pre-merger stage, as indicated by hot X-ray gas bridges between the two merging members. All these properties are highly similar to those of the two proto-typical radio bridge systems already detected with LOFAR. Radio bridges are expected to be detected in the two targets above 3σ with a spatial resolution of 1.5’, assuming that they have the same radio flux density as two proto-typical sources (~ 3 mJy beam-1 at about 140 MHz). The spectral slopes will also be measured with 5 spectral points. By jointly analysing the MWA radio data and the high-quality archival X-ray data acquired with Chandra and XMM-Newton, we will able to study the pre-merger stage dynamics in the virial region, and better constrain the origin and mechanism of the radio bridges.
Searching for the FRB121102 at low radio frequencies
Since the first discovered FRB in 2007, more than 100 sources have been published. The majority of the FRBs are observed not to repeat. However, the discovery of FRB 121102 revealed that at least a subset of FRBs exhibits a repeating nature. FRB 121102 has been studied extensively over the past five years. Recently a trial period P0 = 157 ± 7 day was found. The extension of the periodicity to the future time predicts that the next activity period is from 2020 November 5 to 2021 February 1. Till now, there is no FRB detection at frequencies <300 MHz; a detection of FRB at low radio frequencies would mean a vast science payoff. We expect to detect FRB 121102 with the MWA in the middle of the next active period and compare its low-frequency properties with the FAST observational results. In addition, we will use Machine Learning methods to process the data to increase the potential of detecting weak FRBs.