Fast radio bursts (FRBs) are short-duration radio transients that occur at random times in host galaxies distributed all over the sky. Large field of view instruments can play a critical role in the blind search for rare FRBs. We present a concept for an all-sky FRB monitor using a compact all-sky phased array (CASPA), which can efficiently achieve an extremely large field of view of $\sim10^4$ square degrees. Such a system would allow us to conduct a continuous, blind FRB search covering the entire southern sky. Using the measured FRB luminosity function, we investigate the detection rate for this all-sky phased array and compare the result to a number of other proposed large field-of-view instruments. We predict a rate of a few FRB detections per week and determine the dispersion measure and redshift distributions of these detectable FRBs. This instrument is optimal for detecting FRBs in the nearby Universe and for extending the high-end of the FRB luminosity function through finding ultraluminous events. Additionally, this instrument can be used to shadow the new gravitational-wave observing runs, detect high-energy events triggered from Galactic magnetars and search for other bright, but currently unknown transient signals.

Fast Radio Bursts (FRBs) are millisecond dispersed radio pulses of predominately extra-galactic origin. Although originally discovered at GHz frequencies, most FRBs have been detected between 400 and 800 MHz. Nevertheless, only a handful of FRBs were detected at radio frequencies $\le$400 MHz. Searching for FRBs at low frequencies is computationally challenging due to increased dispersive delay that must be accounted for. Nevertheless, the wide field of view (FoV) of low-frequency telescopes – such as the the Murchison Widefield Array (MWA), and prototype stations of the low-frequency Square Kilometre Array (SKA-Low) – makes them promising instruments to open a low-frequency window on FRB event rates, and constrain FRB emission models. The standard approach, inherited from high-frequencies, is to form multiple tied-array beams to tessellate the entire FoV and perform the search on the resulting time series. This approach, however, may not be optimal for low-frequency interferometers due to their large FoVs and high spatial resolutions leading to a large number of beams. Consequently, there are regions of parameter space in terms of number of antennas and resolution elements (pixels) where interferometric imaging is computationally more efficient. Here we present a new high-time resolution imager BLINK implemented on modern graphical processing units (GPUs) and intended for radio astronomy data. The main goal for this imager is to become part of a fully GPU-accelerated FRB search pipeline. We describe the imager and present its verification on real and simulated data processed to form all-sky and widefield images from the MWA and prototype SKA-Low stations. We also present and compare benchmarks of the GPU and CPU code executed on laptops, desktop computers, and Australian supercomputers. The code is publicly available at https://github.com/PaCER-BLINK-Project/imager and can be applied to data from any radio telescope.

RadioTalk is a communication platform that enabled members of the Radio Galaxy Zoo (RGZ) citizen science project to engage in discussion threads and provide further descriptions of the radio subjects they were observing in the form of tags and comments. It contains a wealth of auxiliary information which is useful for the morphology identification of complex and extended radio sources. In this paper, we present this new dataset, and for the first time in radio astronomy, we combine text and images to automatically classify radio galaxies using a multi-modal learning approach. We found incorporating text features improved classification performance which demonstrates that text annotations are rare but valuable sources of information for classifying astronomical sources, and suggests the importance of exploiting multi-modal information in future citizen science projects. We also discovered over 10000 new radio sources beyond the RGZ-DR1 catalogue in this dataset.

The report of a detection of an absorption profile centred at 78 MHz in the continuum radio background spectrum by the EDGES experiment and its interpretation as the redshifted 21 cm signal of cosmological origin has become one of the most debated results of observational cosmology in recent times. The cosmological 21 cm has long been proposed to be a powerful probe for observing the early Universe and tracing its evolution over cosmic time. Even though the science case is well established, measurement challenges posed on the technical ground are not fully understood to the level of claiming a successful detection. EDGES’s detection has naturally motivated a number of experimental attempts worldwide to corroborate the findings. In this paper, we present a precision cross-correlation spectrometer HYPEREION purpose-designed for a precision radio background measurement between 50–120 MHz to detect the absorption profile reported by the EDGES experiment. HYPEREION implements a pre-correlation signal processing technique that self-calibrates any spurious additive contamination from within the system and delivers a differential measurement of the sky spectrum and a reference thermal load internal to the system. This ensures an unambiguous ‘zero-point’ of absolute calibration of the purported absorption profile. We present the system design, measurement equations of the ideal system, systematic effects in the real system, and finally, an assessment of the real system output for the detection of the absorption profile at 78 MHz in the continuum radio background spectrum.

We describe the design, validation, and commissioning of a new correlator termed ‘MWAX’ for the Murchison Widefield Array (MWA) low-frequency radio telescope. MWAX replaces an earlier generation MWA correlator, extending correlation capabilities and providing greater flexibility, scalability, and maintainability. MWAX is designed to exploit current and future Phase II/III upgrades to MWA infrastructure, most notably the simultaneous correlation of all 256 of the MWA’s antenna tiles (and potentially more in future). MWAX is a fully software-programmable correlator based around an ethernet multicast architecture. At its core is a cluster of 24 high-performance GPU-enabled commercial-off-the-shelf compute servers that together process in real-time up to 24 coarse channels of 1.28 MHz bandwidth each. The system is highly flexible and scalable in terms of the number of antenna tiles and number of coarse channels to be correlated, and it offers a wide range of frequency/time resolution combinations to users. We conclude with a roadmap of future enhancements and extensions that we anticipate will be progressively rolled out over time.

The formation of the first stars and galaxies during ‘Cosmic Dawn’ is thought to have imparted a faint signal onto the 21-cm spin temperature from atomic Hydrogen gas in the early Universe. Observationally, an absorption feature should be measurable as a frequency dependence in the sky-averaged (i.e. global) temperature at meter wavelengths. This signal should be separable from the smooth—but orders of magnitude brighter—foregrounds by jointly fitting a log-polynomial and absorption trough to radiometer spectra. A majority of approaches to measure the global 21-cm signal use radiometer systems on dipole-like antennas. Here, we argue that beamforming-based methods may allow radio arrays to measure the global 21-cm signal. We simulate an end-to-end drift-scan observation of the radio sky at 50–100 MHz using a zenith-phased array, and find that the complex sidelobe structure introduces a significant frequency-dependent systematic. However, the $\lambda/D$ evolution of the beam width with frequency does not confound detection. We conclude that a beamformed array with a median sidelobe level ${\sim}-50$ dB may offer an alternative method to measure the global 21-cm signal. This level is achievable by arrays with $O(10^5)$ antennas.

We present a study of scintillation induced by the mid-latitude ionosphere. By implementing methods currently used in Interplanetary Scintillation studies to measure amplitude scintillation at low frequencies, we have proven it is possible to use the Murchison Widefield Array to study ionospheric scintillation in the weak regime, which is sensitive to structures on scales ${\sim}300$ m at our observing frequency of 154 MHz, where the phase variance on this scale was $0.06\, \textrm{rad}^{2}$ in the most extreme case observed. Analysing over 1000 individual 2-min observations, we compared the ionospheric phase variance with that inferred with previous measurements of refractive shifts, which are most sensitive to scales almost an order of magnitude larger. The two measurements were found to be highly correlated (Pearson correlation coefficient 0.71). We observed that for an active ionosphere, the relationship between these two metrics is in line with what would be expected if the ionosphere’s structure is described by Kolmogorov turbulence between the relevant scales of 300 and 2000 m. In the most extreme ionospheric conditions, the refractive shifts were sometimes found to underestimate the small-scale variance by a factor of four or more, and it is these ionospheric conditions that could have significant effects on radio astronomy observations.

We present the first survey of radio frequency interference (RFI) at the future site of the low frequency Square Kilometre Array (SKA), the Murchison Radio-astronomy Observatory (MRO), that both temporally and spatially resolves the RFI. The survey is conducted in a 1 MHz frequency range within the FM band, designed to encompass the closest and strongest FM transmitters to the MRO (located in Geraldton, approximately 300 km distant). Conducted over approximately three days using the second iteration of the Engineering Development Array in an all-sky imaging mode, we find a range of RFI signals. We are able to categorise the signals into: those received directly from the transmitters, from their horizon locations; reflections from aircraft (occupying approximately 13% of the observation duration); reflections from objects in Earth orbit; and reflections from meteor ionisation trails. In total, we analyse 33 994 images at 7.92 s time resolution in both polarisations with angular resolution of approximately 3.5 $^{\circ}$ , detecting approximately forty thousand RFI events. This detailed breakdown of RFI in the MRO environment will enable future detailed analyses of the likely impacts of RFI on key science at low radio frequencies with the SKA.