arXiv subjects

A. O'Brien

Publications and source records attributed to A. O'Brien.

14 recordsLinked to original sources

arXiv2024

Proton Resonance Shift Thermometry: A review of modern clinical practices

Magnetic Resonance Imaging (MRI) has become a popular modality in guiding minimally invasive thermal therapies, due to its advanced, non-ionizing, imaging capabilities and its ability to record changes in temperature. A variety of MR thermometry techniques have been developed over the years, and Proton Resonance Frequency (PRF) Shift Thermometry is the current clinical gold standard to treat a variety of cancers. It is used extensively to guide hyperthermic thermal ablation techniques such as high-intensity focused ultrasound (HIFU) and laser-induced thermal therapy (LITT). Essential attributes of PRF Shift Thermometry include excellent linearity with temperature, good sensitivity, and independence from tissue type. This non-invasive temperature mapping method gives accurate quantitative measures of the temperature evolution inside biological tissues. In this review, the current status and new developments in the fields of MR-guided HIFU and LITT are presented with an emphasis on breast, prostate, bone, uterine and brain treatments.

physics.med-ph↗

arXiv2022

Radio Transients and Variables in the Tenth Deeper, Wider, Faster Observing Run

The Deeper, Wider, Faster (DWF) program coordinates observations with telescopes across the electromagnetic spectrum, searching for transients on timescales of milliseconds to days. The tenth DWF observing run was carried out in near real-time during September 2021 and consisted of six consecutive days of observations of the NGC 6744 galaxy group and a field containing the repeating fast radio burst FRB190711 with the Australian Square Kilometre Array Pathfinder, the Dark Energy Camera, the Hard X-ray Modulation Telescope and the Parkes 64m "Murriyang" radio telescope. In this work we present the results of an image-domain search for transient, variable and circularly polarised sources carried out with ASKAP using data from the observing run, along with test observations prior to the run and follow-up observations carried out during and after the run. We identified eight variable radio sources, consisting of one pulsar, six stellar systems (five of which exhibit circularly polarised emission) and one previously uncatalogued source. Of particular interest is the detection of pulses from the ultra-cool dwarf SCR J1845-6357 with a period of $14.2\pm 0.3$ h, in good agreement with the known optical rotation period, making this the slowest rotating radio-loud ultra-cool dwarf discovered.

astro-ph.HE↗

arXiv2022

Redshift Evolution of the Feedback / Cooling Equilibrium in the Core of 48 SPT Galaxy Clusters: A Joint $\boldsymbol{Chandra}$-SPT-ATCA analysis

We analyze the cooling and feedback properties of 48 galaxy clusters at redshifts $0.4 < z < 1.3$ selected from the South Pole Telescope (SPT) catalogs to evolve like the progenitors of massive and well-studied systems at $z{\sim}0$. We estimate the radio power at the brightest cluster galaxy (BCG) location of each cluster from an analysis of Australia Telescope Compact Array (ATCA) data. Assuming that the scaling relation between radio power and active galactic nucleus (AGN) cavity power $P_{\mathrm{cav}}$ observed at low redshift does not evolve with redshift, we use these measurements in order to estimate the expected AGN cavity power in the core of each system. We estimate the X-ray luminosity within the cooling radius $L_{\mathrm{cool}}$ of each cluster from a joint analysis of the available $Chandra$ X-ray and SPT Sunyaev-Zel'dovich (SZ) data. This allows us to characterize the redshift evolution of the $P_{\mathrm{cav}} / L_{\mathrm{cool}}$ ratio. When combined with low-redshift results, these constraints enable investigations of the properties of the feedback/cooling cycle across 9~Gyr of cluster growth. We model the redshift evolution of this ratio measured for cool core clusters by a log-normal distribution $\mathrm{Log}$-$\mathcal{N}(α+ βz, σ^2)$ and constrain the slope of the mean evolution $β= -0.05\pm 0.47$. This analysis improves the constraints on the slope of this relation by a factor of two. We find no evidence of redshift evolution of the feedback/cooling equilibrium in these clusters which suggests that the onset of radio-mode feedback took place at an early stage of cluster formation. High values of $P_{\mathrm{cav}} / L_{\mathrm{cool}}$ are found at the BCG location of non-cool core clusters which might suggest that the timescales of the AGN feedback cycle and the cool core / non-cool core transition are different.

astro-ph.CO↗

arXiv2022

Deep ASKAP EMU Survey of the GAMA23 field: Properties of radio sources

We present the Australian Square Kilometre Array Pathfinder (ASKAP) observations of the Galaxy and Mass Assembly (GAMA)-23h field. The survey was carried out at 887.5 MHz and covers a 83 square degree field. We imaged the calibrated visibility data, taken as part of the Evolutionary Mapping of Universe (EMU) Early Science Programme, using the latest version of the ASKAPSoft pipeline. The final mosaic has an angular resolution of 10 arcsec and a central rms noise of around 38 $μ$Jy beam$^{-1}$. The derived radio source catalogue has 39812 entries above a peak flux density threshold of 5$σ$. We searched for the radio source host galaxy counterparts using the GAMA spectroscopic (with an i-band magnitude limit of 19.2 mag) and multi-wavelength catalogues that are available as part of the collaboration. We identified hosts with GAMA spectroscopic redshifts for 5934 radio sources. We describe the data reduction, imaging, and source identification process, and present the source counts. Thanks to the wide area covered by our survey, we obtain very robust counts down to 0.2 mJy. ASKAP's exceptional survey speed, providing efficient, sensitive and high resolution mapping of large regions of the sky in conjunction with the multi-wavelength data available for the GAMA23 field, allowed us to discover 63 giant radio galaxies. The data presented here demonstrate the excellent capabilities of ASKAP in the pre-SKA era.

astro-ph.GA↗

arXiv2021

A comprehensive search for the radio counterpart of GW190814 with the Australian Square Kilometre Array Pathfinder

We present results from a search for the radio counterpart to the possible neutron star-black hole merger GW190814 with the Australian Square Kilometre Array Pathfinder. We have carried out 10 epochs of observation spanning 2-655 days post-merger at a frequency of 944 MHz. Each observation covered 30 deg$^2$, equivalent to 87% of the event localisation. We conducted an untargeted search for radio transients in the field, as well as a targeted search for transients associated with known galaxies. We find one radio transient, ASKAP J005022.3-230349, but conclude that it is unlikely to be associated with the merger. We use our observations to place constraints on the inclination angle of the merger and the density of the surrounding environment by comparing our non-detection to model predictions for radio emission from compact binary coalescences. This survey is also the most comprehensive widefield search (in terms of sensitivity and both areal and temporal coverage) for radio transients to-date and we calculate the radio transient surface density at 944 MHz.

astro-ph.HE↗

arXiv2021

Proton light yield of fast plastic scintillators for neutron imaging

Plastic organic scintillators have been tailored in composition to achieve ultra-fast temporal response, thereby enabling the design and development of fast neutron detection systems with high timing resolution. Eljen Technology's plastic organic scintillators -- EJ-230, EJ-232, and EJ-232Q -- are prospective candidates for use in emerging neutron imaging systems, where fast timing is paramount. To support the neutron response characterization of these materials, the relative proton light yields of EJ-230, EJ-232, and EJ-232Q were measured at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory. Using a broad-spectrum neutron source and a double time-of-flight technique, the proton light yield relations were obtained over a proton recoil energy range of approximately 300 keV to 4 MeV. The EJ-230, EJ-232, and EJ-232Q scintillators exhibited similar proton light yield relations to each other as well as to other plastic scintillators with the same polymer base material. A comparison of the relative proton light yield of different sized cylindrical EJ-232 and EJ-232Q scintillators also revealed consistent results. This work provides key input data for the realistic computational modeling of neutron detection technologies employing these materials, thereby supporting new capabilities in near-field radionuclide detection for national security applications.

physics.ins-det↗

arXiv2021

Remnant Radio Galaxies Discovered in a Multi-frequency Survey

The remnant phase of a radio galaxy begins when the jets launched from an active galactic nucleus are switched off. To study the fraction of radio galaxies in a remnant phase, we take advantage of a $8.31$\,deg$^2$ sub-region of the GAMA~23~field which comprises of surveys covering the frequency range 0.1--9\,GHz. We present a sample of 104 radio galaxies compiled from observations conducted by the Murchison Wide-field Array (216\,MHz), the Australia Square Kilometer Array Pathfinder (887\,MHz), and the Australia Telescope Compact Array (5.5\,GHz). We adopt an `absent radio core' criterion to identify 10 radio galaxies showing no evidence for an active nucleus. We classify these as new candidate remnant radio galaxies. Seven of these objects still display compact emitting regions within the lobes at 5.5\,GHz; at this frequency the emission is short-lived, implying a recent jet switch-off. On the other hand, only three show evidence of aged lobe plasma by the presence of an ultra-steep spectrum ($α<-1.2$) and a diffuse, low surface-brightness radio morphology. The predominant fraction of young remnants is consistent with a rapid fading during the remnant phase. Within our sample of radio galaxies, our observations constrain the remnant fraction to $4\%\lesssim f_{\mathrm{rem}} \lesssim 10\%$; the lower limit comes from the limiting case in which all remnant candidates with hotspots are simply active radio galaxies with faint, undetected radio cores. Finally, we model the synchrotron spectrum arising from a hotspot to show they can persist for 5--10\,Myr at 5.5\,GHz after the jets switch off -- radio emission arising from such hotspots can therefore be expected in an appreciable fraction of genuine remnants.

astro-ph.GA↗

arXiv2020

The Abell 3391/95 galaxy cluster system: A 15 Mpc intergalactic medium emission filament, a warm gas bridge, infalling matter clumps, and (re-) accelerated plasma discovered by combining SRG/eROSITA data with ASKAP/EMU and DECam data

We used dedicated SRG/eROSITA X-ray, ASKAP/EMU radio, and DECam optical observations of a 15 sq.deg region around the interacting galaxy cluster system A3391/95 to study the warm-hot gas in cluster outskirts and filaments, the surrounding large-scale structure and its formation process. We relate the observations to expectations from cosmological hydrodynamic simulations from the Magneticum suite. We trace the irregular morphology of warm-hot gas of the main clusters from their centers out to well beyond their characteristic radii, $r_{200}$. Between the two main cluster systems, we observe an emission bridge; thanks to eROSITA's unique soft response and large field of view, we discover tantalizing hints for warm gas. Several matter clumps physically surrounding the system are detected. For the "Northern Clump," we provide evidence that it is falling towards A3391 from the hot gas morphology and radio lobe structure of its central AGN. Many of the extended sources in the field detected by eROSITA are known clusters or new clusters in the background, including a known SZ cluster at redshift z=1. We discover an emission filament north of the virial radius, $r_{100}$, of A3391 connecting to the Northern Clump and extending south of A3395 towards another galaxy cluster. The total projected length of this continuous warm-hot emission filament is 15 Mpc, running almost 4 degrees across the entire eROSITA observation. The DECam galaxy density map shows galaxy overdensities in the same regions. The new datasets provide impressive confirmation of the theoretically expected structure formation processes on the individual system level, including the surrounding warm-hot intergalactic medium distribution compared to the Magneticum simulation. Our spatially resolved findings show that baryons indeed reside in large-scale warm-hot gas filaments with a clumpy structure.

astro-ph.CO↗

arXiv2020

PKS 2250$-$351: A Giant Radio Galaxy in Abell 3936

We present a detailed analysis of the radio galaxy PKS 2250-351, a giant of 1.2 Mpc projected size, its host galaxy, and its environment. We use radio data from the Murchison Widefield Array, the upgraded Giant Metre-wavelength Radio Telescope, the Australian Square Kilometre Array Pathfinder, and the Australia Telescope Compact Array to model the jet power and age. Optical and infra-red data come from the Galaxy And Mass Assembly (GAMA) survey and provide information on the host galaxy and environment. GAMA spectroscopy confirms that PKS 2250-351 lies at z=0.2115 in the irregular, and likely unrelaxed, cluster Abell 3936. We find its host is a massive, `red and dead' elliptical galaxy with negligible star formation but with a highly obscured active galactic nucleus dominating the mid-infrared emission. Assuming it lies on the local M-sigma relation it has an Eddington accretion rate of lambda_EDD~0.014. We find that the lobe-derived jet power (a time-averaged measure) is an order of magnitude greater than the hotspot-derived jet power (an instantaneous measure). We propose that over the lifetime of the observed radio emission (~300 Myr) the accretion has switched from an inefficient advection dominated mode to a thin-disc efficient mode, consistent with the decrease in jet power. We also suggest that the asymmetric radio morphology is due to its environment, with the host of PKS 2250-351 lying to the west of the densest concentration of galaxies in Abell 3936.

astro-ph.GA↗

arXiv2019

Discovery of a very young high-mass X-ray binary associated with the supernova remnant MCSNRJ0513-6724 in the LMC

We report the discovery of a very young high-mass X-ray binary (HMXB) system associated with the supernova remnant (SNR) MCSNRJ0513-6724 in the Large Magellanic Cloud (LMC), using XMM-Newton X-ray observations. The HMXB is located at the geometrical centre of extended soft X-ray emission, which we confirm as an SNR. The HMXB spectrum is consistent with an absorbed power law with spectral index ~1.6 and a luminosity of 7x10^{33} ergs/s (0.2--12 keV). Tentative X-ray pulsations are observed with a periodicity of 4.4 s and the OGLE I-band light curve of the optical counterpart from more than 17.5 years reveals a period of 2.2324\pm0.0003 d, which we interpret as the orbital period of the binary system. The X-ray spectrum of the SNR is consistent with non-equilibrium shock models as expected for young/less evolved SNRs. From the derived ionisation time scale we estimate the age of the SNR to be <6 kyr. The association of the HMXB with the SNR makes it the youngest HMXB, in the earliest evolutionary stage known to date. A HMXB as young as this can switch on as an accreting pulsar only when the spin period has reached a critical value. Under this assumption, we obtain an upper limit to the magnetic field of < 5x10^{11} G. This implies several interesting possibilities including magnetic field burial, possibly by an episode of post-supernova hyper-critical accretion. Since these fields are expected to diffuse out on a timescale of 10^{3}-10^{4} years, the discovery of a very young HMXB can provide us the unique opportunity to observe the evolution of the observable magnetic field for the first time in X-ray binaries.

astro-ph.HE↗

arXiv2018

A Morphological Study of the Supernova Remnant RX J0852.0-4622 (Vela Jr.)

We conduct a multi-wavelength morphological study of the Galactic supernova remnant RXJ0852.0-4622 (also known as Vela Jr., Vela Z and G266.2-1.2). RX J0852.0-4622 is coincident with the edge of the larger Vela supernova remnant causing confusion in the attribution of some filamentary structures to either RX J0852.0-4622 or its larger sibling. We find that the RX J0852.0-4622 radio continuum emission can be characterised by a 2-dimensional shell with a radius of 0.90+/-0.01deg (or 11.8+/-0.6pc at an assumed distance of 750pc) centred at (l,b)=(133.08+/-0.01 deg,-46.34+/-0.01deg) (or RA=8h52m19.2s, Dec=-46deg20'24.0'', J2000), consistent with X-ray and gamma-ray emission. Although [OIII] emission features are generally associated with the Vela SNR, one particular [OIII] emission feature, which we denote as "the Vela Claw", morphologically matches a molecular clump that is thought to have been stripped by the stellar progenitor of the RX J0852.0-4622 SNR. We argue that the Vela Claw feature is possibly associated with RX J0852.0-4622. Towards the north-western edge of RX J0852.0-4622, we find a flattening of the radio spectral index towards another molecular clump also thought to be associated with RX J0852.0-4622. It is currently unclear whether this feature and the Vela Claw result from interactions between the RX J0852.0-4622 shock and the ISM.

astro-ph.HE↗

arXiv2015

Sub-kiloparsec Imaging of Cool Molecular Gas in Two Strongly Lensed Dusty, Star-Forming Galaxies

We present spatially-resolved imaging obtained with the Australia Telescope Compact Array (ATCA) of three CO lines in two high-redshift gravitationally lensed dusty star-forming galaxies, discovered by the South Pole Telescope. Strong lensing allows us to probe the structure and dynamics of the molecular gas in these two objects, at z=2.78 and z=5.66, with effective source-plane resolution of less than 1kpc. We model the lensed emission from multiple CO transitions and the dust continuum in a consistent manner, finding that the cold molecular gas as traced by low-J CO always has a larger half-light radius than the 870um dust continuum emission. This size difference leads to up to 50% differences in the magnification factor for the cold gas compared to dust. In the z=2.78 galaxy, these CO observations confirm that the background source is undergoing a major merger, while the velocity field of the other source is more complex. We use the ATCA CO observations and comparable resolution Atacama Large Millimeter/submillimeter Array dust continuum imaging of the same objects to constrain the CO-H_2 conversion factor with three different procedures, finding good agreement between the methods and values consistent with those found for rapidly star-forming systems. We discuss these galaxies in the context of the star formation - gas mass surface density relation, noting that the change in emitting area with observed CO transition must be accounted for when comparing high-redshift galaxies to their lower redshift counterparts.

astro-ph.GA↗

arXiv2014

Multi-frequency observations of a superbubble in the LMC: The case of LHA 120-N 70

We present a detailed study of new Australia Telescope Compact Array (ATCA) and XMM-Newton observations of LHA 120-N 70 (hereafter N 70), a spherically shaped object in the Large Magellanic Cloud (LMC) classified as a superbubble (SB). Both archival and new observations were used to produce high quality radio-continuum, X-ray and optical images. The radio spectral index of N 70 is estimated to be $α=-0.12\pm 0.06$ indicating that while a supernova or supernovae have occurred in the region at some time in the distant past, N70 is not the remnant of a single specific supernova. N70 exhibits limited polarisation with a maximum fractional polarisation of 9% in a small area of the north west limb. We estimate the size of N 70 to have a diameter of 104 pc ($\pm 1$ pc). The morphology of N 70 in X-rays closely follows that in radio and optical, with most X-ray emission confined within the bright shell seen at longer wavelengths. Purely thermal models adequately fit the soft X-ray spectrum which lacks harder emission (above 1 keV). We also examine the pressure output of N 70 where the values for the hot (PX) and warm (PHii) phase are consistent with other studied Hii regions. However, the dust-processed radiation pressure (PIR) is significantly smaller than in any other object studied in Lopez et al. (2013). N70 is a very complex region that is likely to have had multiple factors contributing to both the origin and evolution of the entire region.

astro-ph.HE↗

arXiv2014

The long helical jet of the Lighthouse nebula, IGR J11014-6103

Jets from rotation-powered pulsars have so far only been observed in systems moving subsonically trough their ambient medium and/or embedded in their progenitor supernova remnant (SNR). Supersonic runaway pulsars are also expected to produce jets, but they have not been confirmed so far. We investigated the nature of the jet-like structure associated to the INTEGRAL source IGR J11014-6103 (the "Lighthouse nebula"). The source is a neutron star escaping its parent SNR MSH 11-61A supersonically at a velocity exceeding 1000 km/s. We observed the Lighthouse nebula and its jet-like X-ray structure through dedicated high spatial resolution observations in X-rays (Chandra) and radio band (ATCA). Our results show that the feature is a true pulsar's jet. It extends highly collimated over >11pc, displays a clear precession-like modulation, and propagates nearly perpendicular to the system direction of motion, implying that the neutron star's spin axis in IGR J11014-6103 is almost perpendicular to the direction of the kick received during the supernova explosion. Our findings suggest that jets are common to rotation-powered pulsars, and demonstrate that supernovae can impart high kick velocities to misaligned spinning neutron stars, possibly through distinct, exotic, core-collapse mechanisms.

astro-ph.HE↗