Abstract Veloce began operations in tandem with the Transiting Exoplanet Survey Satellite (TESS), at a time marked by rapid growth in the number of known exoplanets. This growth underscores Veloce’s core purpose: the detection of small planets orbiting cool stars. The spectrograph is mounted on the Anglo-Australian Telescope, and is a high-resolution, fibre-fed instrument that offsets the costs of instrument stabilisation onto data analysis. In order to achieve high resolution for the spectrograph, it was necessary to sub-sample the typical telescope seeing with multiple small fibres. Unfortunately, this complex optical design solution causes the images of different fibres to overlap. The overlapping images have limited the radial velocity precision of the spectra. The applicability of Veloce to general spectroscopy has been greatly improved by extending the wavelength range to cover green and blue ranges down to 396 nm. To date, the data are only available as raw images, with some data being custom reduced on a case-by-case basis, but no official general reduction pipeline is available. In this paper, we introduce a new, fully automated, end-to-end pipeline designed for both quick-look assessment during observations and the production of science-ready spectra. The pipeline is publicly available, and while its extraction method is intentionally simplified, the results are consistent and coherent for use in non-radial-velocity science, including stellar characterisation and elemental abundance work.
We present FIRES, a polarised shot-noise (PSN) framework that models fast radio burst (FRB) dynamic spectra as the incoherent superposition of Gaussian microshots with varying polarisation angles (PAs). Applied to the CRAFT bursts FRB 20191001A and FRB 20240318A, FIRES can reproduce key spectro-polarimetric behaviours seen in these data: scattering suppresses PA variability on the trailing edge, while the leading edge preferentially retains intrinsic structure when sufficient signal-to-noise is present. We quantify this behaviour using the PA variance ratio script upper R Subscript psi R psi $\mathcal{R}_\psi$ and explore the joint plane of measured linear polarisation fraction normal upper Pi Subscript upper L Pi L $\Pi_L$ versus PA variance to identify allowed regions of microshot number N, intrinsic PA dispersion sigma Subscript psi sigma psi $\sigma_\psi$ , and intrinsic linear fraction normal upper Pi Subscript upper L comma 0 Pi L , 0 $\Pi_{L,0}$ at fixed signal-to-noise. This restricts these combinations permitted within the adopted shot-noise framework. For FRB 20191001A, the data are consistent with an extended parameter space, reflecting degeneracies between intrinsic PA structure, microshot superposition, scattering, finite sampling, noise, and the assumed microshot-property distributions. FRB 20240318A occupies a more restricted region, favouring fewer microshots and larger intrinsic PA dispersion. By combining an emission-mechanism-independent forward-modeling framework with minimal assumptions and observational constraints, FIRES facilitates qualitative and quantitative exploration of how microshot superposition, scattering, finite sampling, and noise can shape observed FRB polarimetry under the PSN model.
Abstract We present the Galactic Plane Monitor (GPM), a dedicated low-frequency radio transient survey conducted with the Murchison Widefield Array at 200 MHz. The GPM targets an underexplored region of transient parameter space spanning timescales from seconds to weeks, where recent discoveries of unusual Galactic radio sources – including long-period transients and other coherent emitters – suggest a rich and largely uncharted population. Leveraging the MWA’s wide field of view, we carried out 72 observing epochs between June 2022 and March 2025, searching tilde 5 ∼ 5 $\sim5$ 000 sq. deg. of the Southern Galactic plane. We developed a multi-stage processing pipeline to search for variability across a broad range of timescales, from intra-snapshot (4 s to minutes), to inter-snapshot (minutes to hours), and inter-epoch (days to years). This approach combines image differencing, statistical variability metrics, and visual inspection to robustly identify transient candidates while mitigating the effects of ionospheric variability and imaging artefacts. In parallel, we produced deep mosaics using a ‘lucky imaging’ strategy to maximise sensitivity to faint persistent sources. We present the detections of five long-period radio transients, seven eclipsing binary millisecond pulsars, seven intrinsically variable pulsars, and the detection of refractive interstellar scintillation and night-time interplanetary scintillation. The GPM represents a key step toward fully mapping the dynamic low-frequency radio sky and informs the design of next-generation transient surveys with the SKA Observatory.
We develop a general relativistic, one-dimensional model of advection-dominated accretion flows (ADAFs) around Schwarzschild black holes, incorporating the full viscous shear tensor and all components of the heat flux four vector. This framework allows for a consistent treatment of anisotropic conduction in curved space time. Our results highlight the dominant role of radial and temporal heat flux components, while the vertical component, which is commonly emphasised in earlier studies, has negligible impact. Thermal conduction is found to significantly modify the disc structure in inner regions, enhancing thermal pressure and vertical expansion while reducing density. At larger radii, conduction effects fade and disc properties converge. The energy budget remains advection-dominated, with viscous heating exceeding radiative cooling and a substantial portion of energy carried inward by advection. Additionally, stronger inflow and higher angular momentum intensify all thermal processes through enhanced compression and shear. Our results affirm that realistic modelling of ADAFs around black holes demands inclusion of both anisotropic and fully general relativistic effects in heat transport and viscosity. We further show that the accretion flow exhibits transonic behaviour, characterised by a relativistic Mach number that increases inward and crosses unity as the flow approaches the black hole, marking the transition from subsonic to supersonic regimes. Moreover, the weak Coulomb coupling in such radiatively inefficient flows leads naturally to a two-temperature plasma, in which ions and electrons evolve quasi-independently, with the ion component dominating the thermodynamic structure.
Abstract The stellar initial mass function (IMF) is a fundamental ingredient in galaxy evolution, linking observed integrated light to stellar masses, star-formation rates, and chemical enrichment histories. Constraining the full IMF shape beyond the Milky Way remains challenging, as most studies focus either on the low-mass end of quiescent galaxies or the high-mass end of star-forming galaxies. Here, we present the first simultaneous analysis of both ends of the IMF in 214 star-forming galaxies from the Hector survey ( z tilde z ∼ $z \sim$ 0.01–0.07). We estimate the low-mass end slope ( alpha Subscript normal l normal o normal w α l o w $\alpha_{\mathrm{low}}$ ) using a stellar population approach that fits IMF-sensitive absorption features with extended star formation histories, while the high-mass end slope ( alpha Subscript normal h normal i normal g normal h α h i g h $\alpha_{\mathrm{high}}$ ) is derived via the Kennicutt diagnostic, which compares the observed H alpha α $\alpha$ equivalent width and g minus r g − r $g-r$ colour with stellar population synthesis model predictions. We find substantial diversity in IMF shapes, with galaxies spanning combinations of bottom-heavy/light and top-heavy/light slopes. A weak but statistically robust correlation between the low- and high-mass IMF slopes is observed, but partial correlation analysis indicates that this apparent link is largely driven by their mutual dependence on stellar mass and metallicity. Both IMF slopes show significant correlations with stellar mass, star formation activity (traced by H alpha α $\alpha$ luminosity and surface density), and stellar metallicity ([M/H]). In general, higher stellar mass, stronger star formation activity, and higher metallicity are associated with both bottom-heavy and top-heavy IMFs. We find that the full IMF shape seems to be modulated by total stellar mass. Partial correlation analysis reveals that alpha Subscript normal l normal o normal w α l o w $\alpha_{\mathrm{low}}$ is primarily driven by [M/H], whereas alpha Subscript normal h normal i normal g normal h α h i g h $\alpha_{\mathrm{high}}$ is mainly linked to stellar mass and recent star formation. Because alpha Subscript normal l normal o normal w α l o w $\alpha_{\mathrm{low}}$ traces the IMF over long-term averages and alpha Subscript normal h normal i normal g normal h α h i g h $\alpha_{\mathrm{high}}$ captures only recent ( less than or equivalent to 10 ≲ 10 $\lesssim10$ Myr) star formation, the processes shaping each end likely occur over different and possibly decoupled timescales. Our findings challenge the universality of the IMF and emphasise the need for galaxy evolution and stellar population models to incorporate a flexible IMF prescription. Accounting for these variations is essential to build an IMF-consistent picture of galaxy evolution across cosmic time.
Radio recombination line (RRL) maser is a useful tool to study massive star formation regions with ionized gas close to new born massive stars. Masers often show sharp line profiles and/or extreme narrow widths, and high brightness temperatures. However, RRL masers were rarely detected only in several sources. Here we report the detection of sharp line profiles of the RRL H29 α , which can be interpreted as maser candidates, in two sources within W49A, a mini-starburst region in our Galaxy. These observations, conducted with high resolution (∼ 0.03″) using the Atacama Large Millimeter/sub-millimeter Array (ALMA), reveal high brightness temperatures up to ∼ 9000K for H29 α emission in another two sources, which might also be regarded as maser candidates. Additionally, suggestions for efficiently identifying RRL maser candidates are also provided.
We present a new data release from the Rapid ASKAP Continuum Survey (RACS), a widefield snapshot radio survey conducted with the Australian SKA Pathfinder (ASKAP). This data release contains the second RACS epoch to make use of ASKAP's low-frequency band, centred on 887.5 MHz with a bandwidth of 288 MHz, referred to as RACS-low2. This RACS-low2 data release includes both Stokes I and V imaging and catalogue data products covering the whole sky up to a declination of ~+48 degrees. RACS-low2 largely follows the observation footprint of the first low-band epoch, though includes additional coverage in the Northern Hemisphere. The observation scheduling and data processing follow previous RACS epochs, making use of autonomous scheduling and holography-derived primary beam models. The Stokes I and V catalogues are derived from images with a median point-spread function (PSF) of 14.2" times 11.8", and have median root-mean-square noise properties of 195 and 163 micro-Jy/PSF, for the Stokes I and V images, respectively. The consolidated Stokes I catalogue contains 3922151 sources. We also constructed a catalogue of Stokes V sources by measuring Stokes V images at the Stokes I source positions. For the 221 Stokes V measurements above the estimated leakage and detection thresholds we provide likely identifications, including detections of 61 radio stars, 85 pulsars, 43 AGN (many likely to be residual leakage), and one source that is not associated with a known astronomical object. These data products, including calibrated visibilities, images, source lists, and consolidated catalogues, are made publicly available through the CSIRO ASKAP Science Data Archive (CASDA).
Over a hundred fast radio burst (FRB) host galaxies have now been identified, enabling both comparisons of host redshift with FRB dispersion measure to study the cosmological distribution of ionised gas, and analyses of host properties in order to identify FRB progenitors. The standard method for determining the most likely FRB host galaxy in an optical image is the Bayesian framework Probabilistic Association of Transients to their Hosts (PATH), which accounts for uncertainties in the radio localisation, and simplified prior distributions on the host being observable. In this work we extend PATH, incorporating physically-motivated priors that are based on expectations about FRB host galaxy magnitudes. We develop three different models for the apparent r-band magnitude distribution based on an FRB's expected host galaxy redshift, P(m_r|z) and combine these with expectations for redshift based on an FRB's dispersion measure, P(z|DM). We fit the parameters of these prior models using host galaxy candidates for 32 FRBs detected by the Australian SKA Pathfinder (ASKAP) in incoherent sum (ICS) mode by the Commensal Real-time ASKAP Fast Transients (CRAFT) survey. Employing PATH with the new priors on the host magnitudes, we find increased confidence in the most probable hosts of all ASKAP ICS FRB host galaxies. All three models predict similar distributions of FRB host magnitudes at low redshift (z ∼ 0.1), and we confirm previous results that the true FRB host galaxy distribution is fainter than expected for a star-formation-weighted distribution (p-value of 0.12
Searches of unassociated gamma-ray sources in the Fermi-LAT catalogues have led to the discoveries of around a fifth of all known millisecond pulsars (MSPs). These searches have almost exclusively been performed at radio frequencies above 300 MHz, where dispersion and scattering in the interstellar medium are less significant. We report on a shallow survey for pulsars targeting 308 unassociated Fermi-LAT sources in archival Murchison Widefield Array (MWA) observations from the Southern-sky MWA Rapid Two-metre (SMART) pulsar survey at 154 MHz. This is the largest radio survey of unassociated Fermi-LAT sources to date, and only the second to be conducted below 300 MHz after a survey with the Low Frequency Array (LOFAR) that discovered three MSPs. Each source was observed for 20 min by digitally beamforming the MWA tile voltages. Searches were then performed using a new pipeline that implements a semi-coherent dispersion removal scheme for MWA data, enabling greater sensitivities to MSPs than is possible with fully-incoherent dispersion removal (e.g. 2-3 times better sensitivity for dispersion measures between 20-40 pc/cm^3). No new pulsars were identified in the survey, which we attribute to insufficient sensitivity. We estimate flux density limits of approximately 30-220 mJy at 154 MHz (or 0.7-5.2 mJy at 1.4 GHz) for a spin period of 2 ms and a duty cycle of 28
Lunar impact flashes provide a direct means of estimating the flux of centimetre-sized meteoroids impacting the lunar surface. However, 25-60 frames per second imaging typical of most monitoring programs limit the ability to resolve the rapid temporal evolution of the impact process, while the integration of Earthshine background restricts the detection of faint flashes. In this work, we present high-speed observations of lunar impact flashes captured at 200 and 250 FPS using the Zadko Telescope in Western Australia. We resolve the light curves of four confirmed events, revealing complex morphologies, some of which are not well modelled by simple exponential decays. One event was simultaneously detected by a second observer using a 50 FPS system, revealing a significantly faster brightness drop in the high-speed data that cannot be explained by spectral differences alone, indicating temporal integration of the vapour plume and subsequent ejecta. Our data also indicates that the initial flash intensity (representing the vapour plume) exhibits significantly less variance across events than the total luminous energy. Furthermore, we found no statistical correlation between the initial luminous energy and the total integrated energy of the flashes in this data, suggesting that the physical mechanism driving the initial vapour expansion may be physically decoupled from the longer-duration glow driven by the cooling ejecta. High temporal resolution combined with high sensitivity are therefore essential for accurately characterising the physical properties of the impactor and distinguishing the initial vapour plume from the subsequent incandescent cooling phase, although a significantly larger dataset is required to definitively constrain these mechanisms.
We describe the latest iteration of upgrades (designated Phase III) to the Murchison Widefield Array (MWA), in the fourth paper in a series that covers the evolution of the telescope from design concept to initial operational facility, and through two major upgrades. As part of the Phase III upgrade of the MWA, we report the completion of work to design, build, and deploy a new fleet of digital receivers that further optimise the MWA for Epoch of Reionisation observations. These receivers complement existing receivers, such that the MWA now supports the full correlation of all 256 antenna tiles currently in the array. This step releases the MWA from the prior constraint of having to correlate only 128 of the 256 tiles at any given time, which means that the maximum instantaneous sensitivity of the MWA is doubled and the maximum number of interferometric baselines is approximately quadrupled. The upgrade is fundamentally enabled by the new MWAX correlator and various other improvements to the MWA sub-systems. In this paper we describe the new digital receivers and the other improvements that result in the Phase III system. A range of operational benefits arise from the upgrade and scientific flexibility is increased. We also comment on the transition from the MWA to the SKA-Low facility near the end of the decade, including a description of some unique science opportunities utilising joint MWA/SKA-Low data during the Science Verification phase of the SKA-Low Array Assembly 2 (AA2) period.
We present a detailed characterisation of radio frequency interference (RFI) in the 2.4 GHz band around Murriyang, CSIRO's Parkes radio telescope. The dominant sources of interference are Wi-Fi and Bluetooth transmissions. We quantify how the intensity and directionality of this RFI vary with time of day and document its evolution over several years. Although most observers currently discard data within this band, our analysis shows that the interference is confined in both timeand frequency and can be effectively mitigated. Using 10 seconds of 16-bit voltage data recorded during observations of the Vela Pulsar (PSR J0835-4510), we demonstrate that the majority of the channelised data remain unaffected by RFI. We compare three RFI detection and mitigation algorithms and evaluate their relative performance. All methods perform effectively, and any could be implemented in real time to enable productive use of this observing band. A real time implementation would allow the scientific use of this 128 MHz observing band to increase, from almost 70
We investigate how galaxy evolution varies with environment in the nearby Universe by comparing an “average” reference volume in the Southern Galactic Pole (SGP) dataset from to the Nexus region, a dynamically assembling superstructure centred on the Abell 4038 galaxy cluster. We quantify environmental effects using the quenched fraction (f_Q) and the specific star formation rate (sSFR) for the star-forming population, measured as functions of stellar mass and group-scale halo mass from . We decouple the stellar–halo mass dependence, demonstrating that f_Q increases with stellar mass in both field and group environments, while group galaxies show an additional dependence on halo mass. The Nexus exhibits systematic differences relative to the SGP baseline, consistent with increased heterogeneity in accretion histories and pre-processing within a forming superstructure. For star-forming galaxies, the mean log(sSFR) declines strongly with stellar mass and shows additional environment-linked suppression in group-scale halos. Within the Nexus, splitting the sample into three projected radial zones around Abell 4038 shows that environmental regulation is not spatially uniform, driven largely by variations in the sampled halo mass function. Finally, a projected phase-space (PPS) analysis of Abell 4038 links quenching to orbital history within the cluster, though this trend is strongly mass-dependent: low-mass galaxies (log(M_stellar) < 10) show no significant change in f_Q. These results demonstrate that the drivers of galaxy evolution depend jointly on stellar mass, local group halo mass, and location within the surrounding large-scale structure, motivating future large-scale, multi-wavelength cosmic web surveys.
Fast radio bursts (FRBs) are bright (Jansky-level) and short-duration ( tilde similar to $\sim$ 1 ms) flashes of extragalactic origin. Observations of single events have now been complemented by large-area surveys, delivering FRB catalogues and enabling the first population studies. The Northern Cross (NC) radio interferometer is one of the instruments performing observations of FRBs. In this work, we present the Italian Northern Cross Atlas of Radio Transients (INCART), a public platform for the distribution of data products from the NC. INCART makes available to the community the FRBs observed by the NC through manageable frequency-time series datasets and catalogues with best-fit physical parameters. The design of INCART guarantees the possibility of scientific re-analysis of the FRB properties, in view also of future releases of the processing pipeline. Furthermore, INCART focuses on long-term storage optimisation, which is a key aspect of state-of-the-art instrumentation. Public access to the FRB data from the NC maximises the legacy value of the collection, facilitates the synergy with other publicly available catalogues, and fosters research group collaborations.
We report the design and functionality of the Murchison Widefield Array Particle Detector Array (MWA PDA), an array of eight particle scintillation detectors deployed to Inyarrimanha Ilgari Bundara, the Murchison Radio-astronomy Observatory (MRO). The purpose of the instrument is to identify cosmic ray extensive air showers (EAS) occurring over the core of the MWA radio telescope and generate a trigger to allow radio data on the event to be captured and analysed. The system also acts as a pathfinder for a much larger instrument to be deployed in the core of the low-frequency component of the Square Kilometre Array, SKA-Low, by the SKA's ultra-high-energy particles science working group. Here, we describe the instrument and associated infrastructure, which has been verified to comply with the strict radio-frequency emissions requirements of the MRO, and was deployed in November 2024. We present calibration data, which demonstrates the ability of each detector to identify individual atmospheric muons at the expected rate, and we characterise the temperature dependence of the system. We describe a sample of 35 500 EAS identified using multi-detector coincidence over a 13-d period, and show how the detector data can be used to reconstruct the arrival directions and approximate energies of these events. We conclude that the particle detector array can reliably trigger on and reconstruct EAS contained within the $\sim$ $103 imes 90$ m $<^>2$ core region, arriving within 20 $<^>{\circ}$ of zenith, at primary cosmic ray energies above $\sim$ 4 PeV. We have also verified that the detector array can generate triggers, allowing the capture of radio data from the MWA correlator for offline analysis.
Context: BL Lacertae is a blazar known for its high flux variability and occasional broadband flares, the origins of which remain unknown. BL Lacertae was found to be in an extended flaring state in July 2020 which continued until the end of 2021. Aims: The long-term flaring activity makes it an ideal candidate to study its spectral and temporal properties during different flux states. This study explores the X-ray temporal and spectral variability of BL Lacertae. Methods: We analysed five observations of BL Lacertae with the XMM-Newton EPIC instrument taken up to the end of 2021. Temporal properties were investigated using the fractional variability method, minimum variability timescale, and the discrete correlation function. Detailed spectral modelling was performed on the two most variable observations, including an investigation of correlations between the soft (0.3-2.0 keV) and hard (2.0-10.0 keV) energy bands. Results: Out of five observations, two observations were found to be highly variable with F-var=19.16 +/- 0.32 and 6.27 +/- 0.43. The observation taken in 2021 corresponds to the highest flux state. The shortest variability timescale in the 0.3-10 keV band is estimated as 1.24 ks. Assuming the X-ray emission is dominated by the synchrotron process, this variability timescale constrains the size of the emission region. Under the assumption of equipartition between the magnetic field and radiating particles, this implies a magnetic field strength of B approximate to 0.4 G. The spectral analysis reveals a softer-when-brighter trend, which is commonly seen in blazars. We modelled the X-ray spectra with single power-law, log-parabola, and broken power-law models. In most cases, a broken power-law provided the best fit based on corrected Akaike Information Criterion (AICc) statistics, and a strong correlation was observed between the break energy and the source flux. When a thermal blackbody component was added to the model, its temperature also showed a positive correlation with flux in some observations. Conclusions: Our work indicates the complex spectral evolution of BL Lacertae during this flare. The spectral break, interpreted as the cooling break within the synchrotron component, shifts to higher energies with increasing flux. The source consistently displayed softer-when-brighter behaviour. In only one observation were the soft and hard bands found to be significantly correlated. The data suggest a scenario where the peak of the synchrotron emission moves into or across the X-ray band as the source brightens.
We report the detection of a long-period companion to the nearby solar-type star HD 38973 using precision radial-velocity (RV) measurements. The RV data reveal a coherent Keplerian signal with a period of tilde similar to $\sim$ 3 000 days and moderate eccentricity, yielding a minimum mass in the sub-Jovian regime. We complement the RV analysis with astrometric constraints from the Hipparcos-Gaia Catalog of Accelerations (HGCA). Although no significant proper-motion anomaly is detected for HD 38973, the absence of an astrometric signal provides an informative upper limit on the companion mass. By combining the RV posterior with the HGCA likelihood, we rule out high-mass solutions at low inclinations and derive a robust upper bound on the true companion mass. We find the best-fitting true mass to be 0.240 Subscript negative 0.040 Superscript plus 0.102 Baseline upper M Subscript normal upper J normal u normal p 0.240 - 0.040 + 0.102 M J u p $0.240_{-0.040}<^>{+0.102}\,M_{\mathrm{Jup}}$ , on an orbit with period 2 733 Subscript negative 190 Superscript plus 210 2 733 - 190 + 210 $2\,733<^>{+210}_{-190}$ days, making HD 38973b a likely cold Saturn. This study highlights the diagnostic power of astrometric non-detections when combined with precision radial velocities, demonstrating that meaningful constraints on companion masses can be obtained even in the absence of a detected astrometric signal.
Precision geodesy relies on the stability of the International Celestial Reference Frame (ICRF), yet its reference sources, Active Galactic Nuclei (AGN), exhibit intrinsic changes in source structure that can manifest as apparent shifts in their astrometric positions. The high-precision radio measurements used to maintain the ICRF therefore provide a powerful means to investigate the astrophysical mechanisms driving these position changes. In particular, the observed astrometric variability offers a unique opportunity to link positional shifts in AGN to high-energy astrophysical processes. We therefore investigated the relationship between the astrometric positions of ICRF AGN and their gamma-ray emission. We measured the positional offsets of radio cores relative to the third realisation of the ICRF at both S/X (2.3/8.4 GHz) and K (24 GHz) bands and compared them to Fermi-LAT (Large Area Telescope) gamma-ray fluxes within +/- 30 days of the radio observation. Out of an initial sample of 92 radio sources selected for having extensive radio astrometric observations, we identified 57 that met our selection criteria of having sufficient overlapping gamma-ray data points to allow for regression analysis. We find a high incidence of statistically significant (p < 0.05) power-law correlations, with similar to 90% of sources exhibiting this behaviour. The nature of this correlation is complex: we observe both positive and negative correlations, and the sign of the correlation can differ between the two frequency bands for the same source. To explain the correlations, we tested several scenarios, including variable gamma-ray emission locations, changes in nuclear opacity, and variations in jet position angle. Our analysis reveals no single, universally applicable explanation. Instead, the results suggest that the observed correlation is driven by a complex interplay of multiple physical mechanisms, the dominance of which likely varies between sources. A search for time lags between the radio position offsets and gamma-ray fluxes revealed tentative-and highly caveated-evidence for a time-delay in only five sources, with no evidence in other sources. A statistical comparison with the Optical Characteristics of Astrometric Radio Sources (OCARS) catalogue shows that, although our sample is biased towards optically brighter sources with better-constrained astrometric solutions due to their larger number of radio observations, it remains representative of the broader AGN population in terms of redshift distribution.
We present the second data release (DR2) of Spectra and Polarisation in Cutouts of Extragalactic sources from RACS (SPICE-RACS). SPICE-RACS DR2 is derived from the third low-band epoch of the Rapid ASKAP Continuum Survey (RACS-low3) and covers the entire sky from the South celestial pole up to a declination of +49^∘; approximately 87.5
Wide-angle outflows, or winds, are associated with a broad range of astrophysical systems, including protostars, massive stars, X-ray binaries, tidal disruption events (TDEs), luminous fast blue optical transients (LFBOTs), and starburst galaxies. When these winds first “turn on," they inflate a “bubble" into their surroundings, bounded by two shocks and a contact discontinuity, and evolve through distinct adiabatic phases prior to the onset of significant radiative cooling. For sufficiently overdense ejecta, the flow quickly relaxes into an interaction-dominated similarity state at early times and later enters an energy-conserving regime. We present a systematic study of these phases for adiabatic winds expanding into power-law density profiles ρ∝ r^-n with 0 ≤ n ≤ 2. Using analytic scalings together with one-dimensional shock-capturing hydrodynamic simulations, we quantify both the relaxation timescales and the accuracy with which the corresponding similarity solutions reproduce the fluid velocity, density, and pressure throughout the shocked bubble. We show that the interaction solutions are attained within only a few dynamical times and remain valid until the reverse-shocked shell is no longer thin relative to the forward-shocked shell, corresponding in practice to an instantaneous overdensity of order unity. For n < 2, the flow subsequently converges to the generalized energy-conserving scaling R_s ∝ t^3/(5-n), while the special case n=2 exhibits a single persistent similarity state. We discuss the durations and implications of these phases for stellar and galactic outflows, TDEs, and LFBOTs.
