arXiv subjects

D. A. Leahy

Publications and source records attributed to D. A. Leahy.

At least 19 recordsLinked to original sources

arXiv2026

Multipole Functions for Image Analysis II: Equal Area Weighting and Application to Supernova Remnant Images

New basis functions for 2 dimensional (2D) image analysis with a circular boundary (referred to as multipole analysis) are derived which are equal-area weighted. We present open access Python code hosted by GitHub, with which users can apply the multipole analysis to images. The new multipole analysis is applied to a set of 28 supernova remnants (SNRs) which are selected to have both radio and X-ray images, and have been identified as Type Ia or Type CC. Each pair of SNR images (radio and X-ray) was convolved to the same spatial resolution prior to analysis. The resulting multipole radial powers and angular powers, from order 0 to 5, for a given SNR are different for different multipoles and for a given multipole are different between X-ray and radio images. The X-ray radial powers (for orders >0) are larger on average than the radio radial powers (more radial structure in X-rays than radio). The angular powers are smaller than the radial powers on average (more radial structure than angular structure). Comparing Type Ia and Type CC populations, the radial powers (for orders >0) are on average larger for Type CC than Type Ia for X-ray and radio images, with larger difference for X-ray images. The angular powers (for orders >0) are similar between Type Ia and Type CC for both radio and X-ray images.

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arXiv2025

Evolutionary Map of the Universe: A pilot survey to detect high Galactic latitude pulsars in variance images with ASKAP

It has been proposed that radio pulsars can be distinguished from other point-like radio sources in continuum images by their unique interstellar scintillation signatures. Using data from the Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) survey, we conducted a pilot survey of radio pulsars at high Galactic latitude regions via the variance imaging method. Out of approximately 59,800 compact radio sources detected in a ~480 square degree survey area, we identified 20 highly variable sources. Among them, 9 are known pulsars, 1 is a known radio star, 1 is an ultra-long period source, 3 are radio star candidates, and the remaining 6 are pulsar candidates. Notably, we discovered two strongly scintillating pulsars: one with a period of 85.707 ms and a dispersion measure (DM) of 19.4 pc/cm^3, and another with a period of 5.492 ms and a DM of 29.5 pc/cm^3. In addition, a third pulsar was discovered in the variance images, with a period of 14.82 ms and a DM of 39.0 pc/cm^3. This source shows a steep radio spectrum and a high degree of circular polarisation. These results underscore the strong potential of variance imaging for pulsar detection in full EMU and future radio continuum surveys planned with Square Kilometre Array (SKA).

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arXiv2025

The Evolutionary Map of the Universe: A new radio atlas for the southern hemisphere sky

We present the Evolutionary Map of the Universe (EMU) survey conducted with the Australian Square Kilometre Array Pathfinder (ASKAP). EMU aims to deliver the touchstone radio atlas of the southern hemisphere. We introduce EMU and review its science drivers and key science goals, updated and tailored to the current ASKAP five-year survey plan. The development of the survey strategy and planned sky coverage is presented, along with the operational aspects of the survey and associated data analysis, together with a selection of diagnostics demonstrating the imaging quality and data characteristics. We give a general description of the value-added data pipeline and data products before concluding with a discussion of links to other surveys and projects and an outline of EMU's legacy value.

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arXiv2023

EMU/GAMA: Radio detected galaxies are more obscured than optically selected galaxies

We demonstrate the importance of radio selection in probing heavily obscured galaxy populations. We combine Evolutionary Map of the Universe (EMU) Early Science data in the Galaxy and Mass Assembly (GAMA) G23 field with the GAMA data, providing optical photometry and spectral line measurements, together with Wide-field Infrared Survey Explorer (WISE) infrared (IR) photometry, providing IR luminosities and colours. We investigate the degree of obscuration in star forming galaxies, based on the Balmer decrement (BD), and explore how this trend varies, over a redshift range of 0<z<0.345. We demonstrate that the radio detected population has on average higher levels of obscuration than the parent optical sample, arising through missing the lowest BD and lowest mass galaxies, which are also the lower star formation rate (SFR) and metallicity systems. We discuss possible explanations for this result, including speculation around whether it might arise from steeper stellar initial mass functions in low mass, low SFR galaxies.

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arXiv2023

WALLABY Pilot Survey: The diversity of HI structural parameters in nearby galaxies

We investigate the diversity in the sizes and average surface densities of the neutral atomic hydrogen (HI) gas discs in ~280 nearby galaxies detected by the Widefield ASKAP L-band Legacy All-sky Blind Survey (WALLABY). We combine the uniformly observed, interferometric HI data from pilot observations of the Hydra cluster and NGC 4636 group fields with photometry measured from ultraviolet, optical and near-infrared imaging surveys to investigate the interplay between stellar structure, star formation and HI structural parameters. We quantify the HI structure by the size of the HI relative to the optical disc and the average HI surface density measured using effective and isodensity radii. For galaxies resolved by >1.3 beams, we find that galaxies with higher stellar masses and stellar surface densities tend to have less extended HI discs and lower HI surface densities: the isodensity HI structural parameters show a weak negative dependence on stellar mass and stellar mass surface density. These trends strengthen when we limit our sample to galaxies resolved by >2 beams. We find that galaxies with higher HI surface densities and more extended HI discs tend to be more star forming: the isodensity HI structural parameters have stronger correlations with star formation. Normalising the HI disc size by the optical effective radius (instead of the isophotal radius) produces positive correlations with stellar masses and stellar surface densities and removes the correlations with star formation. This is due to the effective and isodensity HI radii increasing with mass at similar rates while, in the optical, the effective radius increases slower than the isophotal radius. Our results demonstrate that with WALLABY we can begin to bridge the gap between small galaxy samples with high spatial resolution HI data and large, statistical studies using spatially unresolved, single-dish data.

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arXiv2022

Physical Properties of the Supernova Remnant Population in the Small Magellanic Cloud

The X-ray emission from a supernova remnant is a powerful diagnostic of the state of its shocked plasma. The temperature and the emission measure are related to the energy of the explosion, the age of the remnant, and the density of the surrounding medium. Here we present the results of a study of the remnant population of the Small Magellanic Cloud. Progress in X-ray observations of remnants has resulted in a sample of 20 remnants in the Small Magellanic Clound with measured temperatures and emission measures. We apply spherically symmetric supernova remnant evolution models to this set of remnants, to estimate ages, explosion energies, and circumstellar medium densities. The distribution of ages yields a remnant birthrate of $\sim$1/1200 yr. The energies and densities are well fit with log-normal distributions, with means of 1.6$\times10^{51}$ erg and 0.14 cm$^{-3}$, and 1$σ$ dispersions of a factor of 1.87 in energy and 3.06 in density, respectively.

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arXiv2021

New Results from the UVIT Survey of the Andromeda Galaxy

The Andromeda Galaxy (M31) has been observed with the UltraViolet Imaging Telescope (UVIT) instrument onboard the AstroSat Observatory. The M31 sky area was covered with 19 fields, in multiple UV filters per field, over the period of 2017 to 2019. The entire galaxy was observed in the FUV F148W filter, and more than half observed in the NUV filters. A new calibration and data processing is described which improves the astrometry and photometry of the UVIT data. The high spatial resolution of UVIT ($\simeq$1 arcsec) and new astrometry calibration ($\simeq$0.2 arcsec) allow identification of UVIT sources with stars, star clusters, X-ray sources, and other source types within M31 to a much better level than previously possible. We present new results from matching UVIT sources with stars measured as part of the Pan-chromatic Hubble Andromeda Treasury project in M31.

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arXiv2020

Results from AstroSat LAXPC Observations of Hercules X-1 (Her X-1)

The Large Area Proportional Counter (LAXPC) instruments onboard the AstroSat Observatory has observed the X-ray binary system Her X-1 during AstroSat observing sessions AO2, AO3 and TO2. These include observations while Her X-1 is in different stages of its 35 day cycle: Low State, Turn-on to Main High State, peak of Main High State and early decline of Main High State. These observations also include a number of dips and one egress of neutron star eclipse. Here we present light curves and softness ratio analysis for these observations and discuss new features of the spectral changes with 35-day phase and orbital phase. We find a new phenomenon for dips during Main High state: About half of the dips show constant softness ratio as count rate decreases, which has not been seen before, and could be caused by highly ionized matter or very dense cold matter. The other half of the dips show the normal decrease of softness ratio as count rate decreases. These are caused by cold matter absorption and were previously known.

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arXiv2020

Evolutionary Models for 43 Galactic Supernova Remnants with Distances and X-ray Spectra

The X-ray emission from a supernova remnant (SNR) is a powerful diagnostic of the state of the shocked plasma. The temperature (kT) and the emission measure (EM) of the shocked-gas are related to the energy of the explosion, the age of the SNR, and the density of the surrounding medium. Progress in X-ray observations of SNRs has resulted in a significant sample of Galactic SNRs with measured kT and EM values. We apply spherically symmetric SNR evolution models to a new set of 43 SNRs to estimate ages, explosion energies, and circumstellar medium densities. The distribution of ages yields a SNR birth rate. The energies and densities are well fit with log-normal distributions, with wide dispersions. SNRs with two emission components are used to distinguish between SNR models with uniform ISM and with stellar wind environment. We find type Ia SNRs to be consistent with a stellar wind environment. Inclusion of stellar wind SNR models has a significant effect on estimated lifetimes and explosion energies of SNRs. This reduces the discrepancy between the estimated SNR birthrate and the SN rate of the Galaxy.

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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.

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arXiv2019

Emission Measures and Emission-measure-weighted Temperatures of Shocked ISM and Ejecta in Supernova Remnants

A goal of supernova remnant (SNR) evolution models is to relate fundamental parameters of a supernova (SN) explosion and progenitor star to the current state of its SNR. The SNR hot plasma is characterized by its observed X-ray spectrum, which yields electron temperature, emission measure and abundances. Depending on their brightness, the properties of the plasmas heated by the SNR forward shock, reverse shock or both can be measured. The current work utilizes models which are spherically symmetric. One dimensional hydrodynamic simulations are carried out for SNR evolution prior to onset of radiative losses. From these, we derive dimensionless emission measures and emission-measure-weighted temperatures, and we present fitting formulae for these quantities as functions of scaled SNR time. These models allow one to infer SNR explosion energy, circumstellar medium density, age, ejecta mass and ejecta density profile from SNR observations. The new results are incorporated into the SNR modelling code SNRPy. The code is demonstrated with application to three historical SNRs: Kepler, Tycho and SN1006.

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arXiv2019

The kinematic distances of SNR G16.7+0.1 and G15.9+0.2 by analyzing HI absorption spectra

We build HI absorption spectra towards Supernova Remnant (SNRs) G16.7+0.1 and G15.9+0.2 using the THOR survey data. With the absorption spectra, we give a new distance range of 7 to 16 kpc for G15.9+0.2. We also resolve the near/far-side distance ambiguity of G16,7+0.1 and confirm its kinematic distance of about 14 kpc. In addition, we analyze the CO (J=3-2) spectra towards G16.7+0.1 and find obvious CO emission at the 20 kms$^{-1}$ OH 1720 MHz maser site. This supports Reynoso and Mangum (2000)'s suggestions that the velocity difference between the maser and southern molecular cloud is caused by the shock acceleration. We discuss the impact of the distances on other physical parameters of the two SNRs.

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arXiv2018

Evolutionary Models for 15 Galactic Supernova Remnants with New Distances

Recent studies using 21 cm HI line and $^{13}$CO line observations in the inner part of the Galaxy have resulted in new distances for 30 Galactic supernova remnants (SNRs). 15 of those remnants have observed X-ray spectra, for which shocked-gas temperatures and emission measures are measured. Here we apply spherically symmetric SNR evolution models to these 15 remnants to obtain estimates for ages, explosion energies, circum-stellar medium densities and profiles (uniform or wind-type).From the distribution of ages we obtain a supernova birth rate and estimate incompleteness. The energies and densities can be well fit with log-normal distributions. The distribution of explosion energies is very similar to that of SNRs in the Large Magellanic Cloud (LMC), suggesting SN explosions in the LMC and in the Galaxy are very similar. The density distribution has higher mean density for Galactic SNRs than for LMC SNRs, by a factor $\sim$2.5.

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arXiv2018

New Distances to Four Supernova Remnants

Distances are found for four supernova remnants without previous distance measurements. H I spectra and H I channel maps are used to determine the maximum velocity of H I absorption for the four supernova remnants (SNRs). We examined $^{13}$CO emission spectra and channel maps to look for possible molecular gas associated with each SNR, but did not find any. The resulting distances for the SNRs are $3.5 \pm 0.2$ kpc (G24.7+0.6), $4.7 \pm 0.3$ kpc (G29.6+0.1), $4.1 \pm 0.5$ kpc (G41.5+0.4) and $4.5 \pm 0 .4 - 9.0 \pm 0.4$ kpc (G57.2+0.8).

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arXiv2018

Distances to Supernova Remnants G$20.4+0.1$, G$24.7-0.6$ and G$28.6-0.1$ and New Molecular Cloud Associations

Accurate distances to supernova remnants (SNRs) are crucial in determining their size, age, luminosity and evolutionary state. To determine distances, we chose three SNRs from the VLA Galactic Plane Survey (VGPS) for extraction of HI absorption spectra. Analysing HI absorption spectra, $^{13}$CO emission spectra, and HI and $^{13}$CO channel maps, kinematic velocities (or their limits) to the three SNRs were calculated. The three SNRs are probably associated with molecular clouds and the new distance to G$20.4+0.1$, G$24.7-0.6$ and G$28.6-0.1$ are $7.8 \pm0.5$ kpc, $3.8\pm 0.2 $ kpc and $ 9.6 \pm 0.3 $ kpc, respectively.

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arXiv2017

Disentangling the radio emission of the supernova remnant W51C

We simulate the evolution of supernova remnant (SNR) W51C. The simulation shows the existence of a new northeast edge. We present magnetic field structure of the W51 complex (SNR W51C and two HII regions W51A/B) by employing the 11 cm survey data of Effelsberg. This new edge is identified and overlaps with W51A along the line of sight, which gives a new angular diameter of about 37' for the quasi-circular remnant. In addition, we assemble the OH spectral lines (1612/1665/1720 MHz) towards the complex by employing the newly released THOR (The HI OH Recombination line survey of Milky Way) data. We find that the known 1720 MHz OH maser in the W51B/C overlap area is located away from the detected 1612/1665MHz absorption region. The latter is sitting at the peak of the HII region G49.2-0.35 within W51B.

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arXiv2017

Energetics and Birth Rates of Supernova Remnants in the Large Magellanic Cloud

Published X-ray emission properties for a sample of 50 supernova remnants (SNRs) in the LMC are used as input for SNR evolution modelling calculations. The forward shock emission is modelled to obtain the initial explosion energy, age, and circumstellar medium density for each SNR in the sample. The resulting age distribution yields a SNR birthrate of 1/(500 yr) for the LMC. The explosion energy distribution is well fit by a log-normal distribution, with most-probable explosion energy of $0.5\times10^{51}$erg, with a 1-$σ$ dispersion by factor 3 in energy. The circumstellar medium density distribution is broader than the explosion energy distribution, with most-probable density of $\sim$0.1 cm$^{-3}$. The shape of the density distribution can be fit with a log-normal distribution, with incompleteness at high density caused by shorter evolution times of SNRs.

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arXiv2017

A 1420 MHz Catalog of Compact Sources in the Northern Galactic Plane

We present a catalog of compact sources of radio emission at 1420 MHz in the northern Galactic plane from the Canadian Galactic Plane Survey. The catalog contains 72,787 compact sources with angular size less than 3$'$ within the Galactic longitude range $52^{\circ} < \ell < 192^{\circ}$ down to a 5$σ$ detection level of $\sim$1.2 mJy. Linear polarization properties are included for 12,368 sources with signals greater than 4$σ_{QU}$ in the CGPS Stokes $Q$ and $U$ images at the position of the total intensity peak. We compare CGPS flux densities with cataloged flux densities in the Northern VLA Sky Survey catalog for 10,897 isolated unresolved sources with CGPS flux density greater than 4 mJy to search for sources that show variable flux density on time scales of several years. We identify 146 candidate variables that exhibit high fractional variations between the two surveys. In addition we identify 13 candidate transient sources that have CGPS flux density above 10\,mJy but are not detected in the NVSS.

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