We present a new radio detection from the Australian Square Kilometre Array Pathfinder Evolutionary Map of the Universe (EMU) survey associated with the Reflection Nebula (RN) VdB-80. The radio detection is determined to be a previously unidentified Hii region, now named Lagotis. The RN is located towards Monoceros, centred in the molecular cloud feature known as the ‘Crossbones’. The 944 MHz EMU image shows a roughly semicircular Hii region with an integrated flux density of 30.2$\pm$0.3 mJy. The Hii region is also seen at 1.4 GHz by NRAO VLA Sky Survey (NVSS), yielding an estimated spectral index of 0.65$\pm$0.51, consistent with thermal radio emission. Gaia Data Release 3 (DR3) and Two Micron All Sky Survey (2MASS) data give a distance to the stars associated with the Hii region of $\sim$960 pc. This implies a size of 0.76$\times$0.68($\pm$0.09) pc for the Hii region. We derive an Hii region electron density of the bright radio feature to be 26 cm$^{-3}$, requiring a Lyman-alpha photon flux of $10^{45.6}$ s$^{-1}$, which is consistent with the expected Lyman flux of HD 46060, the B2 ii type star which is the likely ionising star of the region. The derived distance to this region implies that the Crossbones feature is a superposition of two filamentary clouds, with Lagotis embedded in the far cloud.

We present a re-discovery of G278.94+1.35a as possibly one of the largest known Galactic supernova remnants (SNRs) – that we name Diprotodon. While previously established as a Galactic SNR, Diprotodon is visible in our new Evolutionary Map of the Universe (EMU) and GaLactic and Extragalactic All-sky MWA (GLEAM) radio continuum images at an angular size of $3{{{{.\!^\circ}}}}33\times3{{{{.\!^\circ}}}}23$, much larger than previously measured. At the previously suggested distance of 2.7 kpc, this implies a diameter of 157$\times$152 pc. This size would qualify Diprotodon as the largest known SNR and pushes our estimates of SNR sizes to the upper limits. We investigate the environment in which the SNR is located and examine various scenarios that might explain such a large and relatively bright SNR appearance. We find that Diprotodon is most likely at a much closer distance of $\sim$1 kpc, implying its diameter is 58$\times$56 pc and it is in the radiative evolutionary phase. We also present a new Fermi-LAT data analysis that confirms the angular extent of the SNR in gamma rays. The origin of the high-energy emission remains somewhat puzzling, and the scenarios we explore reveal new puzzles, given this unexpected and unique observation of a seemingly evolved SNR having a hard GeV spectrum with no breaks. We explore both leptonic and hadronic scenarios, as well as the possibility that the high-energy emission arises from the leftover particle population of a historic pulsar wind nebula.

We investigate the unusual H$\alpha$ features found towards the Scutum Supershell via recent arc-minute and arc-second resolution imaging. These multi-degree features resemble a long central spine ending in a bow-shock morphology. We performed a multi–wavelength study in [S II] optical, radio continuum, infrared continuum, Hi, CO, X-ray, and gamma-ray emissions. Interestingly, we found the Galactic worm GW 16.9−3.8 Hi feature appears within the Scutum Supershell, and likely influences the spine morphology. Furthermore, the rightmost edge of the bow-shock H$\alpha$ emission overlaps with [S II] line emission, 4.85 GHz radio, and both 60 and 100 $\mu$m infrared continuum emissions, suggesting some potential for excitation by shock heating. We estimated the photo-ionisation from O-type and B-type stars in the region (including those from the OB associations Ser OB1B, Ser OB2, and Sct OB3) and found that this mechanism could supply the excitation to account for the observed H$\alpha$ luminosity of the spine and bow-shock of $\sim$1–2 $\times 10^{36}\,\mathrm{erg\,s}^{-1}$ (d/2.5 kpc)$^2$. Recent MHD simulations by Drozdov et al. (2022) demonstrate the potential for supernova events to drive outflow and bow-shock types of features of the same energetic nature and physical scale as the H$\alpha$ emission we observe here. While this clearly requires many supernova events over time, we speculate that one contributing event could have come from the presumably energetic supernova (hypernova) birth of the magnetar tentatively identified in the X-ray binary LS 5039.

We present new Australia Telescope Compact Array (ATCA) radio observations towards N 49, one of the brightest extragalactic supernova remnants (SNRs) located in the Large Magellanic Cloud (LMC). Our new and archival ATCA radio observations were analysed along with Chandra X-ray data. These observations show a prominent ‘bullet’ shaped feature beyond the southwestern boundary of the SNR. Both X-ray morphology and radio polarisation analysis support a physical connection of this feature to the SNR. The ‘bullet’ feature’s apparent velocity is estimated at $\sim$1 300 km s$^{-1}$, based on its distance ($\sim$10 pc) from the remnant’s geometric centre and estimated age ($\sim$7 600 yr). we estimated the radio spectral index, $\alpha= -0.55 \pm 0.03$ which is typical of middle-age SNRs. Polarisation maps created for N 49 show low to moderate levels of mean fractional polarisation estimated at 7$\pm$1% and 10$\pm$1% for 5.5 and 9 GHz, respectively. These values are noticeably larger than found in previous studies. Moreover, the mean value for the Faraday rotation of SNR N 49 from combining CABB data is 212$\pm$65 rad m$^{-2}$ and the maximum value of RM is 591$\pm$103 rad m$^{-2}$.

As TeV gamma-ray astronomy progresses into the era of the Cherenkov Telescope Array (CTA), instantaneously following up on gamma-ray transients is becoming more important than ever. To this end, a worldwide network of Imaging Atmospheric Cherenkov Telescopes has been proposed. Australia is ideally suited to provide coverage of part of the Southern Hemisphere sky inaccessible to H.E.S.S. in Namibia and the upcoming CTA-South in Chile. This study assesses the sources detectable by a small, transient-focused array in Australia based on CTA telescope designs. The TeV emission of extragalactic sources (including the majority of gamma-ray transients) can suffer significant absorption by the extragalactic background light. As such, we explored the improvements possible by implementing stereoscopic and topological triggers, as well as lowered image cleaning thresholds, to access lower energies. We modelled flaring gamma-ray sources based on past measurements from the satellite-based gamma-ray telescope Fermi-LAT. We estimate that an array of four Medium-Sized Telescopes (MSTs) would detect $\sim$24 active galactic nucleus flares >5$\sigma$ per year, up to a redshift of $z\approx1.5$. Two MSTs achieved $\sim$80–90% of the detections of four MSTs. The modelled Galactic transients were detectable within the observation time of one night, 11 of the 21 modelled gamma-ray bursts were detectable, as were $\sim$10% of unidentified transients. An array of MST-class telescopes would thus be a valuable complementary telescope array for transient TeV gamma-ray astronomy.

We report the discovery of a bow-shock pulsar wind nebula (PWN), named Potoroo, and the detection of a young pulsar J1638$-$4713 that powers the nebula. We present a radio continuum study of the PWN based on 20-cm observations obtained from the Australian Square Kilometre Array Pathfinder (ASKAP) and MeerKAT. PSR J1638$-$4713 was identified using Parkes radio telescope observations at frequencies above 3 GHz. The pulsar has the second-highest dispersion measure of all known radio pulsars (1 553 pc cm$^{-3}$), a spin period of 65.74 ms and a spin-down luminosity of $\dot{E}=6.1\times10^{36}$ erg s$^{-1}$. The PWN has a cometary morphology and one of the greatest projected lengths among all the observed pulsar radio tails, measuring over 21 pc for an assumed distance of 10 kpc. The remarkably long tail and atypically steep radio spectral index are attributed to the interplay of a supernova reverse shock and the PWN. The originating supernova remnant is not known so far. We estimated the pulsar kick velocity to be in the range of 1 000–2 000 km s$^{-1}$ for ages between 23 and 10 kyr. The X-ray counterpart found in Chandra data, CXOU J163802.6$-$471358, shows the same tail morphology as the radio source but is shorter by a factor of 10. The peak of the X-ray emission is offset from the peak of the radio total intensity (Stokes $\rm I$) emission by approximately 4.7$^{\prime\prime}$, but coincides well with circularly polarised (Stokes $\rm V$) emission. No infrared counterpart was found.

As TeV gamma-ray astronomy progresses into the era of the Cherenkov Telescope Array (CTA), there is a desire for the capacity to instantaneously follow up on transient phenomena and continuously monitor gamma-ray flux at energies above $10^{12}\,\mathrm{eV}$ . To this end, a worldwide network of Imaging Air Cherenkov Telescopes (IACTs) is required to provide triggers for CTA observations and complementary continuous monitoring. An IACT array sited in Australia would contribute significant coverage of the Southern Hemisphere sky. Here, we investigate the suitability of a small IACT array and how different design factors influence its performance. Monte Carlo simulations were produced based on the Small-Sized Telescope (SST) and Medium-Sized Telescope (MST) designs from CTA. Angular resolution improved with larger baseline distances up to 277 m between telescopes, and energy thresholds were lower at 1 000 m altitude than at 0 m. The ${\sim} 300\,\mathrm{GeV}$ energy threshold of MSTs proved more suitable for observing transients than the ${\sim}1.2\,\mathrm{TeV}$ threshold of SSTs. An array of four MSTs at 1 000 m was estimated to give a 5.7 $\sigma$ detection of an RS Ophiuchi-like nova eruption from a 4-h observation. We conclude that an array of four MST-class IACTs at an Australian site would ideally complement the capabilities of CTA.

The Galactic TeV ${\gamma}$ -ray source ${\mathrm{HESS\,J}1804{-}216}$ is currently an unidentified source. In an attempt to unveil its origin, we present here the most detailed study of interstellar gas using data from the Mopra Southern Galactic Plane CO Survey, 7- and 12-mm wavelength Mopra surveys and Southern Galactic Plane Survey of HI. Several components of atomic and molecular gas are found to overlap ${\mathrm{HESS\,J}1804{-}216}$ at various velocities along the line of sight. The CS(1-0) emission clumps confirm the presence of dense gas. Both correlation and anti-correlation between the gas and TeV ${\gamma}$ -ray emission have been identified in various gas tracers, enabling several origin scenarios for the TeV ${\gamma}$ -ray emission from ${\mathrm{HESS\,J}1804{-}216}$ . For a hadronic scenario, ${\mathrm{SNR\,G}8.7{-}0.1}$ and the progenitor supernova remnant (SNR) of ${\mathrm{PSR\,J}1803{-}2137}$ require cosmic ray (CR) enhancement factors of ${\mathord{\sim} 50}$ times the solar neighbour CR flux value to produce the TeV ${\gamma}$ -ray emission. Assuming an isotropic diffusion model, CRs from both these SNRs require a slow diffusion coefficient, as found for other TeV SNRs associated with adjacent ISM gas. The morphology of gas located at 3.8 kpc (the dispersion measure distance to ${\mathrm{PSR\,J}1803{-}2137}$ ) tends to anti-correlate with features of the TeV emission from ${\mathrm{HESS\,J}1804{-}216}$ , making the leptonic scenario possible. Both pure hadronic and pure leptonic scenarios thus remain plausible.

We present an update on the Mopra Central Molecular Zone Carbon Monoxide (CO) survey, with data taken in 2016 extending the original 3.5° ≥ l ≥ 358.5°, +1.0° ≥ b ≥ -0.5° to 4.0° ≥ l ≥ 358.0°, +1.0° ≥ b ≥ -1.0°. Using the four simultaneously observed lines of 12CO, 13CO, C18O, and C17O Nyquist sampled at 0.6′ spatial and 0.1 km/s spectral resolution, we are building an optical-thickness-corrected three-dimensional model of the diffuse gas, and making cloud mass estimates. This data, as part of the Mopra Southern Galactic Plane CO Survey (Braiding et al. (2015), Burton et al. (2013)), is at the highest resolution available across such a widespread region, and includes the Sagittarius A, Sagittarius B2, Sagittarius C, and G1.3 cloud complexes, as well as Bania’s Clump 2.

We present results from a Mopra 7 mm-wavelength survey that targeted the dense gas-tracing CS(1-0) transition towards the young γ-ray-bright supernova remnant, RX J1713.7–3946 (SNR G 347.3−0.5). In a hadronic γ-ray emission scenario, where cosmic ray (CR) protons interact with gas to produce the observed γ-ray emission, the mass of potential CR target material is an important factor. We summarise newly discovered dense gas components, towards Cores G and L, and Clumps N1, N2, N3, and T1, which have masses of 1 – 104 M⊙. We argue that these components are not likely to contribute significantly to γ-ray emission in a hadronic γ-ray emission scenario. This would be the case if RX J1713.7–3946 were at either the currently favoured distance of ~1 kpc or an alternate distance (as suggested in some previous studies) of ~6 kpc.

This survey also targeted the shock-tracing SiO molecule. Although no SiO emission corresponding to the RX J1713.7–3946 shock was observed, vibrationally excited SiO(1-0) maser emission was discovered towards what may be an evolved star. Observations taken 1 yr apart confirmed a transient nature, since the intensity, line-width, and central velocity of SiO(J = 1-0,v = 1,2) emission varied significantly.