Epoch of Reionisation – Centaurus A and Galactic Plane
Detection of the signal of spatial fluctuations in the 21cm brightness temperature in the early Universe will open the door to exploration of this crucial era. The signal structure and evolution encode key astrophysical and cosmological information, which will allow an understanding of the first sources of ionising photons, the growth of structure, and the physical conditions in the intergalactic medium. The MWA Epoch of Reionisation project currently has some of the strongest limits in the research field, and we look forward to applying our analysis to EoR data with the new MWAX correlator. Prior to the normal EoR season, which commences in August, we propose to observe Centaurus A and the Galactic Plane with MWAX in May-June for commissioning observations. Cen A is a complex source that causes systematics in our EoR2 field data, and an improved model for this would be valuable. The Galactic Plane can be used for EoR calibration.
MWA Observations of the Sun
Thirty hours of observing time for solar observations is requested during the 2021A 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 to 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.
Rapid-response MWA observations of Swiftgamma-ray bursts
We request the use of the MWA rapid-response mode to perform triggered VCS observations of Swift gamma-ray bursts (GRBs) during the 2021A semester. The prompt and early-time radio emission associated with GRBs is still a poorly explored regime, particularly at MHz frequencies. There are two main classes of GRBs: short and long. Short GRBs (SGRBs)are linked with binary neutron star (BNS)mergers and predicted to 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. Long GRBs (LGRBs) are linked with core-collapse supernovae, and could also generate low-frequency prompt radio emission (Katz, 2016; Falcke & Rezzolla, 2014), though it remains unclear whether this emission can escape from the local environment (Zhang, 2014). 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 20 seconds of receiving an alert, MWA is uniquely capable of being on-target in time to observe the earliest prompt emission. The MWA VCS observation with its high temporal resolution is most suitable for searching for the prompt radio emission associated with GRBs. We focus on Swift alerts, which, unlike Fermi alerts, provide ~arcsec localisations for GRBs. This enables us to localise the GRB to a synthesised beam of MWA and coherently beam form the VCS data, which can maximize our sensitivity. Even with non-detection, such rapid-response MWA VCS 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.
Follow-up of a periodic radio transient
Using data from the GaLactic and Extragalactic All-sky MWA eXtended (GLEAM-X) survey, we have detected a bright, periodic, highly polarised radio transient. It was switched on for the period of January 2018 to March 2018 inclusive, and despite an exhaustive search, and follow-up DDT in early 2021, we have been unable to find detections outside of this period. We request further follow-up time to establish whether the transient has reappeared, which will allow us to obtain high signal-to-noise light curves, examine it for any changes in periodicity, and determine the viability of proposing for VCS time.
Science commissioning of the high-time resolution mode of MWAX and advancing the SMART pulsar survey
This proposal seeks telescope time with the two-pronged objective of undertaking the science com-missioning of the new high-time resolution mode of MWAX and taking advantage of the window of opportunity provided by the availability of the compact array configuration for advancing the SMART pulsar survey and bringing the related data collection effort to completion. The SMART survey is an all-sky pulsar search project with the MWA with the eventual goal of surveying the entire sky south of+30deg in declination. It exploits the MWA’s large field of view and the newly developed multi-pixel functionality of the software beamformer. With the discovery of the first new pulsar from the processing of only a small fraction of data, the project clearly hints at an exciting future for low-frequency pulsar searches in the southern hemisphere. Follow-up efforts with the uGMRT and Parkes confirmed the steep-spectrum low-luminosity nature of the pulsar. A publication reporting this discovery is currently in press. The imminent transition to the MWAX era means the final phase of the SMART observing campaign will need to be undertaken as a shared-risk project, and thus necessitates the commissioning of the new VCS functionality that will be available through the MWAX system. While the SMART project will indeed benefit from the full range of capabilities and functionalities that we will test, validate and commission, the overall exercise is also critically important for continued science exploitation of the MWA for pulsar work, another overarching goal of this project.
Time-domain science: pulsars
Commissioning the voltage dump mode of MWAX for triggering on ASKAP FRB detections
This proposal aims to undertake the science commissioning of the voltage dump functionality of MWAX, in response to external triggers from other facilities such as ASKAP. This is important for the continued exploitation of the MWA for low-frequency triggering on FRBs detected with other facilities. The field of FRBs has enviably flourished over the past half decade, following significant breakthroughs including an explosion in their tally with CHIME coming online, and the first interferometric localisation with ASKAP. Measurements of their scattering, scintillation, polarisation and Faraday rotation properties are becoming routine, and theoretical efforts to understand their origin and progenitors continue unabated. With its large field of view (~30deg^2) and interferometric advantages, ASKAP has emerged as the front runner in the field, with multiple FRBs localised to arcsecond (or better) precision. The MWA’s co-location at the Murchison site provides unique opportunities for undertaking efficient co-observing and triggering observations. Our shadowing campaigns in early years (2018-2019) yielded the most stringent constraints on the low-frequency emission of apparently non-repeating FRBs and their spectral indices. Starting the 2020A semester, we shifted the focus to the development and testing of VCS trigger modes and strategies and optimising them. Through this proposal we will interface, test and commission the VCS trigger capability of the new high time resolution mode. 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. While this proposal is primarily centred around triggering on ASKAP FRBs, we note that similar projects are being conceived in coordination with FAST and UTMOST, and thus the proposed commissioning is vital for a broader range of transient projects with the MWA.
Time-domain science: transients
7.5 (+ engineering test time)
Monitoring of X-ray binary transient outbursts with the MWA
We propose pointed observations with the MWA of any nearby bright (>300 mJy) outbursting X-ray binary (XRB) during the 2021-A observing semester. The low-frequency regime of radio jets in XRBs is still not fully explored, especially at frequencies <500MHz. XRBs can produce two different types of radio jets over the course of a single outburst. Optically thick flat-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 transient jets using MWA observations to measure the radio spectrum in the low-frequency band. Simultaneous high-cadence monitoring of transient jets at low and high frequencies (possibly with ASKAP, MeerKAT, ATCA, and LBA) will provide observational measurements essential for constraining the low-frequency emission geometry and thereby constrain various theoretical jet models. To this end, we aim to provide high-quality low-frequency radio light curves of XRBs covering denser sampling over the hard-to-soft state transition.
Rapid-response MWA observations of UTMOST-2D Fast Radio Bursts
We request the use of the MWA rapid-response mode to perform triggered VCS observations on fast radio bursts (FRBs) detected by the recently-upgraded UTMOST-2D radio telescope. The origin of FRBs remains a mystery, and there is debate as to whether there are two different FRB populations; repeaters and non-repeaters. While the majority of bursts have been detected around~1GHz, there are now reported detections (from a repeating FRB) with LOFAR between 110-188 MHz. Observations of FRBs at low frequencies are important as they allow us to place constraints on the circumburst medium and help us to understand the FRB emission mechanism. For example, is the lack of low frequency (<200 MHz)FRB detections due to free-free absorption or is it a clue about the burst physics? Could FRBs be broadband or do we expect a single emitter to produce signals that have an individually narrow band-width but occurring over the entire radio frequency range? To perform FRB searches with the MWA, the high time resolution of the VCS mode is required to detect these narrow-band (ms) signals. However, blind searches are not viable for the MWA VCS as the data collection rate is 28TB/hr and a dedispersion search on this data volume is not computationally feasible. Instead, we propose to perform triggered VCS observations from FRBs detected with UTMOST-2D. For a typical FRBDM range of 300-3000 pc cm^-3, the response time ofUTMOST-2D transmitting an alert and MWA repointing is less than the expected dispersion delay between the observing frequencies of the two telescopes. Taking into account this delay along with the expected dispersion sweep across the MWA band, we only require 5 minutes of VCS data capture per trigger to search for a signal via coherent beam-forming at the exact known FRB position provided by UTMOST-2D.