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James A. Green

Publications and source records attributed to James A. Green.

At least 19 recordsLinked to original sources

SKAO and Gamma-Ray Synergies

A wide variety of Galactic and extragalactic sources are known to emitradiation across the entire electromagnetic spectrum, including both transient and steady-state phenomena. A few hundred of these sources (~300) have been detected even at the highest energies, in the TeV range. The number of known TeV emitters is expected to increase substantially in the coming years with the operation of current and next-generation Cherenkov detectors, such as the Large High Altitude Air Shower Observatory (LHAASO) and the Cherenkov Telescope Array Observatory (CTAO). These sources typically exhibit broad, non-thermal, spectral energy distributions. Explaining such emission requires efficient particle acceleration mechanisms (e.g. Fermi processes, shock acceleration) and radiative processes involving magnetic fields (e.g. synchrotron and inverse Compton radiation), often accompanied by polarization signatures. However, the relative contribution of these emission mechanisms and the underlying physical processes are still debated. In this work, we present an overview of the scientific potential arising from the synergy between the Square Kilometre Array (SKA) and current and upcoming gamma-ray facilities. Combined observations across these energy bands will provide crucial insights into the physical mechanisms driving emission from GeV-TeV sources of both Galactic and extragalactic origin. These include transient events (e.g. gamma-ray bursts, supernovae, fast radio bursts, tidal disruption events, neutrino and gravitational-wave counterparts), variable sources (e.g. blazars, active galactic nuclei), and steady emitters (e.g. the Galactic centre, supernova remnants, radio galaxies, and galaxy clusters). We discuss the prospects for coordinated SKA-gamma-ray observations, including wide-field surveys, monitoring of variable sources, and target-of-opportunity follow-ups.

astro-ph.HE

Measuring Magnetic Fields Near and Far with the SKA via the Zeeman Effect

Zeeman splitting in spectral lines -- both in emission and absorption -- provides direct estimates of magnetic field strength and direction in magnetized gas in our own Milky Way and in external galaxies. We discuss the potential for using the Square Kilometre Array (SKA) to measure the Zeeman effect in targets spanning an enormous range of distance: from cometary comas in the solar system, through Galactic molecular clouds, HI filaments in the cold neutral medium, high-velocity clouds, the Fermi Bubbles, and photodissociation regions (PDRs) traced by radio recombination lines, to OH masers and megamasers in nearby and distant starburst galaxies, and to cold neutral gas in damped Ly-alpha absorbing systems at cosmological redshifts. We update the sensitivity calculations of Robishaw et al. (2015) and indicate, for each science goal, whether it will be achievable with Array Assembly 4 (AA4) of SKA-Mid, with the staged delivery of AA*, or only with the full SKA buildout. Zeeman measurements will probe the magnetic field in situ in the warm and cold neutral interstellar medium, complementing SKA Faraday rotation programs; radio recombination lines, stackable across hundreds of transitions, extend this reach to HII regions and PDRs. In external galaxies, SKA-Mid will enable Zeeman studies of OH kilomasers in nearby starburst systems, substantially expand the census of megamaser Zeeman detections beyond the Arecibo sky, and probe magnetic fields in damped Lyman-alpha systems to field limits well below those currently achievable, opening a new window on the role of magnetic fields in galaxy formation and cosmic evolution.

astro-ph.GA

Measuring Magnetic Field Strengths in Galactic Star-forming Regions via the Zeeman Effect with the SKA

Magnetic fields thread the interstellar medium from the largest to the smallest scales and play an important role in molecular cloud evolution and star formation. Quantifying this requires measurements of the field strengths, and the most direct way to measure them is via the Zeeman effect in spectral lines. The effect is subtle for the typical field strengths expected from theory, from a few $μ$G in diffuse molecular clouds to a few 10s of mG in dense star-forming regions, and detections are scarce. Existing measurements of magnetic field strength suggest dense clouds and cores are marginally supercritical (cannot prevent collapse, but can inhibit it), but may be biased due to small sample sizes. Zeeman effect measurements tracing different scales and densities within molecular clouds can reveal the variation of field strengths, providing critical measurements to address the question of whether star formation is primarily regulated by magnetic fields or turbulence on different scales. Observations with SKA precursors such MeerKAT and FAST are beginning to increase the number of Zeeman effect detections in nearby star-forming regions. The SKA will extend their reach to many regions within our Galaxy that are best representative of where most stars form, while zooming in on the densest star-forming regions, providing a statistical basis for the role of magnetic fields in molecular cloud evolution and star formation. We present predictions and plans for Zeeman effect observations with the SKA telescopes, demonstrating the significant advances they will provide for studies of magnetic fields in molecular clouds.

astro-ph.GA

Roadmap on Advancements of the FHI-aims Software Package

Electronic-structure theory is the foundation of the description of materials including multiscale modeling of their properties and functions. Obviously, without sufficient accuracy at the base, reliable predictions are unlikely at any level that follows. The software package FHI-aims has proven to be a game changer for accurate free-energy calculations because of its scalability, numerical precision, and its efficient handling of density functional theory (DFT) with hybrid functionals and van der Waals interactions. It treats molecules, clusters, and extended systems (solids and liquids) on an equal footing. Besides DFT, FHI-aims also includes quantum-chemistry methods, descriptions for excited states and vibrations, and calculations of various types of transport. Recent advancements address the integration of FHI-aims into an increasing number of workflows and various artificial intelligence (AI) methods. This Roadmap describes the state-of-the-art of FHI-aims and advancements that are currently ongoing or planned.

cond-mat.mtrl-sci

Tiny-Scale Properties within the Interstellar Medium towards PSR J1644$-$4559: I. Observational Evidence of Turbulence-induced Tiny Scale Atomic Structures

We investigated HI absorption toward a single pulsar, PSR J1644$-$4559, and its variability over timescales from days to years, using Murriyang, CSIRO's Parkes Radio Telescope. Our 19 epochs of spectral observations, spanning 1.2 years with intervals as short as 1 day, provide the most comprehensive cadence coverage for monitoring HI absorption to date. We identified two significant detections of tiny-scale atomic structure (TSAS) with spatial scales ranging from a lower limit of $\sim$11 au to an upper limit of 165 au, both exhibiting integrated signal-to-noise ratios exceeding 5.0. We find a relationship between linear size and optical depth variation in the cold neutral medium (CNM) component hosting the TSAS, described by a power-law relationship, $Δτ_{\rm int} = Δτ_0 (ΔL)^{(α-2)/2}$, with $α= 4.1 \pm 0.4$. This is the first observational evidence explicitly connecting TSAS to turbulence in CNM. This power-law index is significantly steeper than previously reported values for the CNM, where $α$ ranges from 2.3 to 2.9, but similar to those observed in the warm ionized gas. Additionally, we observe no significant variation in $α$ across the entire range of spatial scales traced in our study, indicating that turbulence may be cascading down and dissipating at smaller scales. While there is no precise proper motion measurement for this pulsar, our estimates for the turbulence dissipation in the CNM place the lower and upper limits at less than 0.03 au and 0.4 au, respectively.

astro-ph.GA

Sciences with Thai National Radio Telescope

This White Paper summarises potential key science topics to be achieved with Thai National Radio Telescope (TNRT). The commissioning phase has started in mid 2022. The key science topics consist of "Pulsars and Fast Radio Bursts (FRBs)", "Star Forming Regions (SFRs)", "Galaxy and Active Galactic Nuclei (AGNs)", "Evolved Stars", "Radio Emission of Chemically Peculiar (CP) Stars", and "Geodesy", covering a wide range of observing frequencies in L/C/X/Ku/K/Q/W-bands (1-115 GHz). As a single-dish instrument, TNRT is a perfect tool to explore time domain astronomy with its agile observing systems and flexible operation. Due to its ideal geographical location, TNRT will significantly enhance Very Long Baseline Interferometry (VLBI) arrays, such as East Asian VLBI Network (EAVN), Australia Long Baseline Array (LBA), European VLBI Network (EVN), in particular via providing a unique coverage of the sky resulting in a better complete "uv" coverage, improving synthesized-beam and imaging quality with reducing side-lobes. This document highlights key science topics achievable with TNRT in single-dish mode and in collaboration with VLBI arrays.

astro-ph.IM

A Low Frequency Pilot Survey of Southern HII Regions in the Vela Constellation

Atomic ionised regions with strong continuum emission are often associated with regions of high-mass star formation and low-frequency (<2GHz) observations of these regions are needed to help build star formation models. The region toward the Vela Supernova Remnant is particularly interesting as it is a complex structure of recent supernova explosions and molecular clouds containing a number of HII regions that are not well characterised. We searched publicly available catalogues for HII regions, both candidate and identified, which also have low-frequency emission. In the area of ~400 square degrees toward the Vela Supernova remnant, we found 10 such HII regions, some of which have multiple components in catalogues. In this work we use data from the Australian Square Kilometre Array Pathfinder and previously unpublished data from the Murchison Widefield Array and the Australian Telescope Compact Array to analyse these sources. The high-mass star forming region RCW 38, with observations specifically targeted on the source, is used as a pilot study to demonstrate how low-frequency, wide-field continuum observations can identify and study HII regions in our Galaxy. For the 9 other HII regions, we discuss their properties; including information about which clouds are interacting, their ages, whether they are dominated by infrared or optical H$α$ lines, distances, ionising photon flux, and upper limits on the infrared luminosity. In future work, these 9 regions will be analysed in more detail, similar to the result for RCW 38 presented here.

astro-ph.GA

Analyzing the Intrinsic Magnetic Field in the Galactic Center Radio Arc

The Radio Arc is a system of organized non-thermal filaments (NTFs) located within the Galactic Center (GC) region of the Milky Way. Recent observations of the Radio Arc NTFs revealed a magnetic field which alternates between being parallel and rotated with respect to the orientation of the filaments. This pattern is in stark contrast to the predominantly parallel magnetic field orientations observed in other GC NTFs. To help elucidate the origin of this pattern, we analyze spectro-polarimetric data of the Radio Arc NTFs using an Australian Telescope Compact Array data set covering the continuous frequency range from $\sim$4 to 11 GHz at a spectral resolution of 2 MHz. We fit depolarization models to the spectral polarization data to characterize Faraday effects along the line-of-sight. We assess whether structures local to the Radio Arc NTFs may contribute to the unusual magnetic field orientation. External Faraday effects are identified as the most likely origin of the rotation observed for the Radio Arc NTFs; however, internal Faraday effects are also found to be likely in regions of parallel magnetic field. The increased likelihood of internal Faraday effects in parallel magnetic field regions may be attributed to the effects of structures local to the GC. One such structure could be the Radio Shell local to the Radio Arc NTFs. Future studies are needed to determine whether this alternating magnetic field pattern is present in other multi-stranded NTFs, or is a unique property resulting from the complex interstellar region local to the Radio Arc NTFs.

astro-ph.GA

First Search for Low-Frequency CH with a Square Kilometre Array Precursor Telescope

The diatomic free radical methylidyne (CH) is an important tracer of the interstellar medium and the study of it was critical to our earliest understanding of star formation. Although it is detectable across the electromagnetic spectrum, observations at radio frequencies allow for a study of the kinematics of the diffuse and dense gas in regions of new star formation. There are only two published (single-dish) detections of the low-frequency hyperfine transitions between 700 and 725 MHz, despite the precise frequencies being known. These low-frequency transitions are of particular interest as they are shown in laboratory experiments to be more sensitive to magnetic fields than their high-frequency counterparts (with more pronounced Zeeman splitting). In this work we take advantage of the radio quiet environment and increased resolution of the Australian Square Kilometre Array Pathfinder (ASKAP) over previous searches to make a pilot interferometric search for CH at 724.7883 MHz (the strongest of the hyperfine transitions) in RCW 38. We found the band is clean of radio frequency interference, but we did not detect the signal from this transition to a five sigma sensitivity limit of 0.09 Jy, which corresponds to a total column density upper limit of 1.9x10^18cm^-2 for emission and 1.3x10^14cm^-2 for absorption with an optical depth limit of 0.95. Achieved within 5 hrs of integration, this column density sensitivity should have been adequate to detect the emission or absorption in RCW 38, if it had similar properties to the only previous reported detections in W51.

astro-ph.GA

Nitric Oxide and other molecules: Molecular Modelling and Low FrequencyExploration using the Murchison Widefield Array

We present new molecular modelling for 14NO and 15NO and a deep, blind molecular line survey at low radio frequencies (99-129 MHz). This survey is the third in a series completed with the Murchison Widefield Array (MWA), but in comparison with the previous surveys, uses four times more data (17 hours vs. 4 hours) and is three times better in angular resolution (1' vs. 3'). The new molecular modelling for nitric oxide and its main isotopologue has seven transitions within the MWA frequency band (although we also present the higher frequency transitions). Although we did not detect any new molecular lines at a limit of 0.21 Jy beam^-1, this work is an important step in understanding the data processing challenges for the future Square Kilometre Array (SKA) and places solid limits on what is expected in the future of low-frequency surveys. The modelling can be utilised for future searches of nitric oxide.

astro-ph.GA

Magnetism Science with the Square Kilometre Array

The Square Kilometre Array (SKA) will answer fundamental questions about the origin, evolution, properties, and influence of magnetic fields throughout the Universe. Magnetic fields can illuminate and influence phenomena as diverse as star formation, galactic dynamics, fast radio bursts, active galactic nuclei, large-scale structure, and Dark Matter annihilation. Preparations for the SKA are swiftly continuing worldwide, and the community is making tremendous observational progress in the field of cosmic magnetism using data from a powerful international suite of SKA pathfinder and precursor telescopes. In this contribution, we revisit community plans for magnetism research using the SKA, in the light of these recent rapid developments. We focus in particular on the impact that new radio telescope instrumentation is generating, thus advancing our understanding of key SKA magnetism science areas, as well as the new techniques that are required for processing and interpreting the data. We discuss these recent developments in the context of the ultimate scientific goals for the SKA era.

astro-ph.GA

The Breakthrough Listen Search for Intelligent Life: Observations of 1327 Nearby Stars over 1.10-3.45 GHz

Breakthrough Listen (BL) is a ten-year initiative to search for signatures of technologically capable life beyond Earth via radio and optical observations of the local Universe. A core part of the BL program is a comprehensive survey of 1702 nearby stars at radio wavelengths (1-10 GHz). Here, we report on observations with the 64-m CSIRO Parkes radio telescope in New South Wales, Australia, and the 100-m Robert C. Byrd Green Bank radio telescope in West Virginia, USA. Over 2016 January to 2019 March, a sample of 1138 stars was observed at Green Bank using the 1.10-1.90 GHz and 1.80-2.80 GHz receivers, and 189 stars were observed with Parkes over 2.60-3.45 GHz. We searched these data for the presence of engineered signals with Doppler-acceleration drift rates between -4 to 4 Hz/s. Here, we detail our data analysis techniques and provide examples of detected events. After excluding events with characteristics consistent with terrestrial radio interference, we are left with zero candidates. Given the sensitivity of our observations, we can put an upper limit on the power of potential radio transmitters at these frequencies at 2x10^12 W, and 9x10^12 W for GBT and Parkes respectively. These observations constitute the most comprehensive search over 1.10-3.45 GHz for technosignatures to date. All data products, totalling ~219 TB, are available for download as part of the first BL data release (DR1), as described in a companion paper (Lebofsky et. al., 2019)

astro-ph.EP

Accurate OH Maser Positions From The SPLASH Survey III: The Final 96 Square Degrees

We present high spatial resolution observations of ground-state OH masers achieved with the Australia Telescope Compact Array (ATCA). These observations targeted 253 pointing centres containing OH maser candidates at all four ground-state OH transitions identified in the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH) across 96 square degrees of the Southern Galactic plane (332\degree$<l<$334\degree and $-$2\degree$<b<+$2\degree, 344\degree$<l<$355\degree and $-$2\degree$<b<+$2\degree, 358\degree$<l<$4\degree and $+$2\degree$<b<+$6\degree, 5\degree$<l<$10\degree and $-$2\degree$<b<+$2\degree). We detect maser emission towards 236 fields and suggest that 7 out of 17 non-detections are due to the slightly lower sensitivity of the ATCA observations, combined with some temporal variability. The superior resolution provided by the ATCA data has allowed us to identify 362 OH maser sites in the 236 target fields. Almost half (160 of 362) of these masers have been detected for the first time. Comparison between these 362 maser sites with information presented in the literature allowed us to categorize 238 sites as evolved star sites (66\%), 63 as star formation (17\%), eight as supernova remnants and 53 unknown maser sites (15\%). We present analysis of the OH masers across the full SPLASH survey range (176 square degrees) and find that the detection rate of 1.7 GHz radio continuum sources (18\%) is lower than that previously found at 8.2 and 9.2 GHz (38\%). We also find that the velocity separations of evolved star sites with symmetric 1612 MHz maser profiles are generally smaller than those with asymmetric profiles.

astro-ph.GA

Simulation of the Quantum Dynamics of Indistinguishable Bosons with the Coupled Coherent States Method

Computer simulations of many-body quantum dynamics of indistinguishable particles is a challenging task for computational physics. In this paper we demonstrate that the method of coupled coherent states (CCS) developed previously for multidimensional quantum dynamics of distinguishable particles can be used to study indistinguishable bosons in the second quantisation formalism. To prove its validity, the technique termed here coupled coherent states for indistinguishable bosons (CCSB) is tested on two model problems. The first is a system-bath problem consisting of a tunnelling mode coupled to a harmonic bath, previously studied by CCS and other methods in distinguishable representation in 20 dimensions. The harmonic bath is comprised of identical oscillators, and may be second quantised for use with CCSB, so that this problem may be thought of as a bosonic bath with an impurity. The cross-correlation function for the dynamics of the system and Fourier transform spectrum compare extremely well with a benchmark calculation, which none of the prior methods of studying the problem achieved. The second model problem involves 100 bosons in a shifted harmonic trap. Breathing oscillations in the 1-body density are calculated and shown to compare favourably to a multiconfigurational time-dependent Hartree for bosons calculation, demonstrating the applicability of the method as a new formally exact way to study the quantum dynamics of Bose-Einstein condensates.

quant-ph

The Breakthrough Listen Search for Intelligent Life: Wide-bandwidth Digital Instrumentation for the CSIRO Parkes 64-m Telescope

Breakthrough Listen is a ten-year initiative to search for signatures of technologies created by extraterrestrial civilizations at radio and optical wavelengths. Here, we detail the digital data recording system deployed for Breakthrough Listen observations at the 64-m aperture CSIRO Parkes Telescope in New South Wales, Australia. The recording system currently implements two recording modes: a dual-polarization, 1.125 GHz bandwidth mode for single beam observations, and a 26-input, 308-MHz bandwidth mode for the 21-cm multibeam receiver. The system is also designed to support a 3 GHz single-beam mode for the forthcoming Parkes ultra-wideband feed. In this paper, we present details of the system architecture, provide an overview of hardware and software, and present initial performance results.

astro-ph.IM

Accurate OH maser positions II. the Galactic Center region

We present high spatial resolution observations of ground-state OH masers, achieved using the Australia Telescope Compact Array (ATCA). These observations were conducted towards 171 pointing centres, where OH maser candidates were identified previously in the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH) towards the Galactic Center region, between Galactic longitudes of $355^{\circ}$ and $5^{\circ}$ and Galactic latitudes of $-2^{\circ}$ and $+2^{\circ}$. We detect maser emission towards 162 target fields and suggest that 6 out of 9 non-detections are due to intrinsic variability. Due to the superior spatial resolution of the follow-up ATCA observations, we have identified 356 OH maser sites in the 162 of the target fields with maser detections. Almost half (161 of 356) of these maser sites have been detected for the first time in these observations. After comparing the positions of these 356 maser sites to the literature, we find that 269 (76\%) sites are associated with evolved stars (two of which are planetary nebulae), 31 (9\%) are associated with star formation, four are associated with supernova remnants and we were unable to determine the origin of the remaining 52 (15\%) sites. Unlike the pilot region (\citealt{Qie2016a}), the infrared colors of evolved star sites with symmetric maser profiles in the 1612 MHz transition do not show obvious differences compared with those of evolved star sites with asymmetric maser profiles.

astro-ph.GA

ALMA imaging of the nascent planetary nebula IRAS 15103-5754

We present continuum and molecular line (CO, C$^{18}$O, HCO$^+$) observations carried out with the Atacama Large Millimeter/submillimeter Array toward the "water fountain" star IRAS 15103-5754, an object that could be the youngest PN known. We detect two continuum sources, separated by $0.39\pm 0.03$ arcsec. The emission from the brighter source seems to arise mainly from ionized gas, thus confirming the PN nature of the object. The molecular line emission is dominated by a circumstellar torus with a diameter of $\simeq 0.6$ arcsec (2000 au) and expanding at $\simeq 23$ km s$^{-1}$. We see at least two gas outflows. The highest-velocity outflow (deprojected velocities up to 250 km s$^{-1}$), traced by the CO lines, shows a biconical morphology, whose axis is misaligned $\simeq 14^\circ$ with respect to the symmetry axis of the torus, and with a different central velocity (by $\simeq 8$ km s$^{-1}$). An additional high-density outflow (traced by HCO$^+$) is oriented nearly perpendicular to the torus. We speculate that IRAS 15103-5754 was a triple stellar system that went through a common envelope phase, and one of the components was ejected in this process. A subsequent low-collimation wind from the remaining binary stripped out gas from the torus, creating the conical outflow. The high velocity of the outflow suggests that the momentum transfer from the wind was extremely efficient, or that we are witnessing a very energetic mass-loss event.

astro-ph.SR

A dearth of OH/IR stars in the Small Magellanic Cloud

We present the results of targeted observations and a survey of 1612-, 1665-, and 1667-MHz circumstellar OH maser emission from asymptotic giant branch (AGB) stars and red supergiants (RSGs) in the Small Magellanic Cloud (SMC), using the Parkes and Australia Telescope Compact Array radio telescopes. No clear OH maser emission has been detected in any of our observations targeting luminous, long-period, large-amplitude variable stars, which have been confirmed spectroscopically and photometrically to be mid- to late-M spectral type. These observations have probed 3 - 4 times deeper than any OH maser survey in the SMC. Using a bootstrapping method with LMC and Galactic OH/IR star samples and our SMC observation upper limits, we have calculated the likelihood of not detecting maser emission in any of the two sources considered to be the top maser candidates to be less than 0.05%, assuming a similar pumping mechanism as the LMC and Galactic OH/IR sources. We have performed a population comparison of the Magellanic Clouds and used Spitzer IRAC and MIPS photometry to confirm that we have observed all high luminosity SMC sources that are expected to exhibit maser emission. We suspect that, compared to the OH/IR stars in the Galaxy and LMC, the reduction in metallicity may curtail the dusty wind phase at the end of the evolution of the most massive cool stars. We also suspect that the conditions in the circumstellar envelope change beyond a simple scaling of abundances and wind speed with metallicity.

astro-ph.SR