Blazars are a subclass of active galactic nuclei with extreme observation properties, which is caused by the beaming effect, expressed by a Doppler factor ( $\delta$ ), in a relativistic jet. Doppler factor is an important parameter in the blazars paradigm to indicate all of the observation properties, and many methods were proposed to estimate its value. In this paper, we present a method following Mattox et al. to calculate the lower limit on $\gamma$ -ray Doppler factor ( $\delta_{\gamma}$ ) for 809 selected Fermi/LAT-detected $\gamma$ -ray blazars by adopting the available $\gamma$ -ray and X-ray data. Our sample included 342 flat-spectrum radio quasars (FSRQs) and 467 BL Lac objects (BL Lacs), out of which 507 sources are compiled with available radio core-dominance parameter (R) from our previous study. Our calculation shows that the average values of the lower limit on $\delta_{\gamma}$ for FSRQs and BL Lacs are $\left\langle\delta_{\gamma}|_{\textrm{FSRQ}}\right\rangle = 6.87 \pm 4.07$ and $\left\langle\delta_{\gamma}|_{\textrm{BL\ Lac}}\right\rangle=4.31 \pm 2.97$ , respectively. We compare and discuss our results with those from the literature. We found that the derived lower limit on $\delta_{\gamma}$ for some sources is higher than that from the radio estimation, which could be possibly explained by the jet bending within those blazars. Our results also suggest that the $\gamma$ -ray and radio regions perhaps share the same relativistic effects. The $\gamma$ -ray Doppler factor has been found to be correlated with both the $\gamma$ -ray luminosity and core-dominance parameter, implying that the jet is possibly continuous in the $\gamma$ -ray bands, and R is perhaps an indicator for a beaming effect.

Early science observations from the Australian Square Kilometre Array Pathfinder (ASKAP) have revealed clear signals of diffuse radio emission associated with two clusters detected by the South Pole Telescope via their Sunyaev Zel’dovich signal: SPT CLJ0553-3342 (MACS J0553.4-3342) and SPT CLJ0638-5358 (Abell S0592) are both high-mass lensing clusters that have undergone major mergers. To create science-fidelity images of the galaxy clusters, we performed direction-dependent (DD) calibration and imaging on these ASKAP early science observations using state-of-the-art software killMS and DDFacet. Here, we present our DD calibrated ASKAP radio images of both clusters showing unambiguous giant radio halos with largest linear scales of ${\sim}1$ Mpc. The halo in MACS J0553.4-3342 was previously detected with Giant Metrewave Radio Telescope observations at 323 MHz but appears more extended in our ASKAP image. Although there is a shock detected in the thermal X-ray emission of this cluster, we find that the particle number density in the shocked region is too low to allow for the generation of a radio shock. The radio halo in Abell S0592 is a new discovery, and the Southwest border of the halo coincides with a shock detected in X-rays. We discuss the origins of these halos considering both the hadronic and turbulent re-acceleration models and sources of seed electrons. This work gives a positive indication of the potential of ASKAP’s Evolutionary Map of the Universe survey in detecting intracluster medium radio sources.

We present a detailed discussion of the implementation strategies for a recently developed w-stacking w-projection hybrid algorithm used to reconstruct wide-field interferometric images. In particular, we discuss the methodology used to deploy the algorithm efficiently on a supercomputer via use of a Message Passing Interface (MPI) k-means clustering technique to achieve efficient construction and application of non-coplanar effects. Additionally, we show that the use of conjugate symmetry can increase the w-stacking efficiency, decrease the time required to construction, and apply w-projection kernels for large data sets. We then demonstrate this implementation by imaging an interferometric observation of Fornax A from the Murchison Widefield Array (MWA). We perform an exact non-coplanar wide-field correction for 126.6 million visibilities using 50 nodes of a computing cluster. The w-projection kernel construction takes only 15 min prior to reconstruction, demonstrating that the implementation is both fast and efficient.

Close double neutron stars (DNSs) have been observed as Galactic radio pulsars, while their mergers have been detected as gamma-ray bursts and gravitational wave sources. They are believed to have experienced at least one common envelope episode (CEE) during their evolution prior to DNS formation. In the last decades, there have been numerous efforts to understand the details of the common envelope (CE) phase, but its computational modelling remains challenging. We present and discuss the properties of the donor and the binary at the onset of the Roche lobe overflow (RLOF) leading to these CEEs as predicted by rapid binary population synthesis models. These properties can be used as initial conditions for detailed simulations of the CE phase. There are three distinctive populations, classified by the evolutionary stage of the donor at the moment of the onset of the RLOF: giant donors with fully convective envelopes, cool donors with partially convective envelopes, and hot donors with radiative envelopes. We also estimate that, for standard assumptions, tides would not circularise a large fraction of these systems by the onset of RLOF. This makes the study and understanding of eccentric mass-transferring systems relevant for DNS populations.

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.

Diffuse, non-thermal emission in galaxy clusters is increasingly being detected in low-frequency radio surveys and images. We present a new diffuse, steep-spectrum, non-thermal radio source within the cluster Abell 1127 found in survey data from the Murchison Widefield Array (MWA). We perform follow-up observations with the ‘extended’ configuration MWA Phase II with improved resolution to better resolve the source and measure its low-frequency spectral properties. We use archival Very Large Array S-band data to remove the discrete source contribution from the MWA data, and from a power law model fit we find a spectral index of –1.83±0.29 broadly consistent with relic-type sources. The source is revealed by the Giant Metrewave Radio Telescope at 150 MHz to have an elongated morphology, with a projected linear size of 850 kpc as measured in the MWA data. Using Chandra observations, we derive morphological estimators and confirm quantitatively that the cluster is in a disturbed dynamical state, consistent with the majority of phoenices and relics being hosted by merging clusters. We discuss the implications of relying on morphology and low-resolution imaging alone for the classification of such sources and highlight the usefulness of the MHz to GHz radio spectrum in classifying these types of emission. Finally, we discuss the benefits and limitations of using the MWA Phase II in conjunction with other instruments for detailed studies of diffuse, steep-spectrum, non-thermal radio emission within galaxy clusters.

Following the results of our previous low-frequency searches for extraterrestrial intelligence (SETI) using the Murchison Widefield Array (MWA), directed towards the Galactic Centre and the Orion Molecular Cloud (Galactic Anticentre), we report a new large-scale survey towards the Vela region with the lowest upper limits thus far obtained with the MWA. Using the MWA in the frequency range 98–128 MHz over a 17-h period, a $400\,\textrm{deg}^{2}$ field centred on the Vela Supernova Remnant was observed with a frequency resolution of 10 kHz. Within this field, there are six known exoplanets. At the positions of these exoplanets, we searched for narrow-band signals consistent with radio transmissions from intelligent civilisations. No unknown signals were found with a 5 $\sigma$ detection threshold. In total, across this work plus our two previous surveys, we have now examined 75 known exoplanets at low frequencies. In addition to the known exoplanets, we have included in our analysis the calculation of the Effective Isotropic Radiated Power (EIRP) upper limits towards over 10 million stellar sources in the Vela field with known distances from Gaia (assuming a 10-kHz transmission bandwidth). Using the methods of Wright, Kanodia, & Lubar (2018) to describe an eight-dimensional parameter space for SETI searches, our survey achieves the largest search fraction yet, two orders of magnitude higher than the previous highest (our MWA Galactic Anticentre survey), reaching a search fraction of $\ \sim2\,{\times}\,10^{-16}$ . We also compare our results to previous SETI programs in the context of the $\mbox{EIRP}_{\textrm{min}}$ —Transmitter Rate plane. Our results clearly continue to demonstrate that SETI has a long way to go. But, encouragingly, the MWA SETI surveys also demonstrate that large-scale SETI surveys, in particular for telescopes with a large field-of-view, can be performed commensally with observations designed primarily for astrophysical purposes.