arXiv subjects

Tara Murphy

Publications and source records attributed to Tara Murphy.

At least 145 records · Page 8Linked to original sources

arXiv2013

High velocity clouds in the Galactic All Sky Survey I. Catalogue

We present a catalogue of high-velocity clouds (HVCs) from the Galactic All Sky Survey (GASS) of southern-sky neutral hydrogen, which has 57 mK sensitivity and 1 km/s velocity resolution and was obtained with the Parkes Telescope. Our catalogue has been derived from the stray-radiation corrected second release of GASS. We describe the data and our method of identifying HVCs and analyse the overall properties of the GASS population. We catalogue a total of 1693 HVCs at declinations < 0 deg, including 1111 positive velocity HVCs and 582 negative velocity HVCs. Our catalogue also includes 295 anomalous velocity clouds (AVCs). The cloud line-widths of our HVC population have a median FWHM of ~19 km/s, which is lower than found in previous surveys. The completeness of our catalogue is above 95% based on comparison with the HIPASS catalogue of HVCs, upon which we improve with an order of magnitude in spectral resolution. We find 758 new HVCs and AVCs with no HIPASS counterpart. The GASS catalogue will shed an unprecedented light on the distribution and kinematic structure of southern-sky HVCs, as well as delve further into the cloud populations that make up the anomalous velocity gas of the Milky Way.

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arXiv2013

Two populations of gamma-ray burst radio afterglows

The detection rate of gamma-ray burst (GRB) afterglows is only ~30% at radio wavelengths, much lower than in the X-ray (~95%) or optical (~70%) bands. The cause of this low radio detection rate has previously been attributed to limited observing sensitivity. We use visibility stacking to test this idea, and conclude that the low detection rate is instead due to two intrinsically different populations of GRBs, radio bright and radio faint. We calculate that no more than 70% of GRB afterglows are truly radio bright, leaving a significant population of GRBs that lack a radio afterglow. These radio bright GRBs have higher gamma-ray fluence, isotropic energies, X-ray fluxes and optical fluxes than the radio faint GRBs, confirming the existence of two physically distinct populations. We suggest that the gamma-ray efficiency of the prompt emission is responsible for the difference between the two populations. We also discuss the implications for future radio and optical surveys.

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arXiv2013

G306.3-0.9: A newly discovered young galactic supernova remnant

We present X-ray and radio observations of the new Galactic supernova remnant (SNR) G306.3-0.9, recently discovered by Swift. Chandra imaging reveals a complex morphology, dominated by a bright shock. The X-ray spectrum is broadly consistent with a young SNR in the Sedov phase, implying an age of 2500 yr for a distance of 8 kpc, plausibly identifying this as one of the 20 youngest Galactic SNRs. Australia Telescope Compact Array (ATCA) imaging reveals a prominent ridge of radio emission that correlates with the X-ray emission. We find a flux density of ~ 160 mJy at 1 GHz, which is the lowest radio flux recorded for a Galactic SNR to date. The remnant is also detected at 24microns, indicating the presence of irradiated warm dust. The data reveal no compelling evidence for the presence of a compact stellar remnant.

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arXiv2013

Science with the Murchison Widefield Array

Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the Southern Hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21 cm emission from the epoch of reionisation in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives.

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arXiv2012

Limits on prompt, dispersed radio pulses from gamma-ray bursts

We have searched for prompt radio emission from nine Gamma Ray Bursts (GRBs) with a 12 m telescope at 1.4 GHz, with a time resolution of 64 us to 1 s. We detected single dispersed radio pulses with significances >6 sigma in the few minutes following two GRBs. The dispersion measures of both pulses are well in excess of the expected Galactic values, and the implied rate is incompatible with known sources of single dispersed pulses. The arrival times of both pulses also coincide with breaks in the GRB X-ray light curves. A null trial and statistical arguments rule out random fluctuations as the origin of these pulses with >95% and 97% confidence, respectively, although a simple population argument supports a GRB origin with confidence of only 2%. We caution that we cannot rule out RFI as the origin of these pulses. If the single pulses are not related to the GRBs we set an upper limit on the flux density of radio pulses emitted between 200 to 1800 s after a GRB of 1.27 w^{-1/2} Jy, where 64 us < w < 32 ms is the pulse width. We set a limit of less than 760 Jy for long timescale (>1 s) variations. These limits are some of the most constraining at high time resolution and GHz frequencies in the early stages of the GRB phenomenon.

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arXiv2012

VAST: An ASKAP Survey for Variables and Slow Transients

The Australian Square Kilometre Array Pathfinder (ASKAP) will give us an unprecedented opportunity to investigate the transient sky at radio wavelengths. In this paper we present VAST, an ASKAP survey for Variables and Slow Transients. VAST will exploit the wide-field survey capabilities of ASKAP to enable the discovery and investigation of variable and transient phenomena from the local to the cosmological, including flare stars, intermittent pulsars, X-ray binaries, magnetars, extreme scattering events, interstellar scintillation, radio supernovae and orphan afterglows of gamma ray bursts. In addition, it will allow us to probe unexplored regions of parameter space where new classes of transient sources may be detected. In this paper we review the known radio transient and variable populations and the current results from blind radio surveys. We outline a comprehensive program based on a multi-tiered survey strategy to characterise the radio transient sky through detection and monitoring of transient and variable sources on the ASKAP imaging timescales of five seconds and greater. We also present an analysis of the expected source populations that we will be able to detect with VAST.

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arXiv2012

BLOBCAT: Software to Catalogue Flood-Filled Blobs in Radio Images of Total Intensity and Linear Polarization

We present BLOBCAT, new source extraction software that utilises the flood fill algorithm to detect and catalogue blobs, or islands of pixels representing sources, in two-dimensional astronomical images. The software is designed to process radio-wavelength images of both Stokes I intensity and linear polarization, the latter formed through the quadrature sum of Stokes Q and U intensities or as a byproduct of rotation measure synthesis. We discuss an objective, automated method by which estimates of position-dependent background root-mean-square noise may be obtained and incorporated into BLOBCAT's analysis. We derive and implement within BLOBCAT corrections for two systematic biases to enable the flood fill algorithm to accurately measure flux densities for Gaussian sources. We discuss the treatment of non-Gaussian sources in light of these corrections. We perform simulations to validate the flux density and positional measurement performance of BLOBCAT, and we benchmark the results against those of a standard Gaussian fitting task. We demonstrate that BLOBCAT exhibits accurate measurement performance in total intensity and, in particular, linear polarization. BLOBCAT is particularly suited to the analysis of large survey data. The BLOBCAT software, supplemented with test data to illustrate its use, is available at: http://blobcat.sourceforge.net/ .

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arXiv2012

Is the observed high-frequency radio luminosity distribution of QSOs bimodal?

The distribution of QSO radio luminosities has long been debated in the literature. Some argue that it is a bimodal distribution, implying that there are two separate QSO populations (normally referred to as 'radio-loud' and 'radio-quiet'), while others claim it forms a more continuous distribution characteristic of a single population. We use deep observations at 20 GHz to investigate whether the distribution is bimodal at high radio frequencies. Carrying out this study at high radio frequencies has an advantage over previous studies as the radio emission comes predominantly from the core of the AGN, hence probes the most recent activity. Studies carried out at lower frequencies are dominated by the large scale lobes where the emission is built up over longer timescales (10^7-10^8 yrs), thereby confusing the sample. Our sample comprises 874 X-ray selected QSOs that were observed as part of the 6dF Galaxy Survey. Of these, 40% were detected down to a 3 sigma detection limit of 0.2-0.5 mJy. No evidence of bimodality is seen in either the 20 GHz luminosity distribution or in the distribution of the R_20 parameter: the ratio of the radio to optical luminosities traditionally used to classify objects as being either radio-loud or radio-quiet. Previous results have claimed that at low radio luminosities, star formation processes can dominate the radio emission observed in QSOs. We attempt to investigate these claims by stacking the undetected sources at 20 GHz and discuss the limitations in carrying out this analysis. However, if the radio emission was solely due to star formation processes, we calculate that this corresponds to star formation rates ranging from ~10 solar masses/yr to ~2300 solar masses/yr.

astro-ph.CO↗

arXiv2012

Compact continuum source-finding for next generation radio surveys

We present a detailed analysis of four of the most widely used radio source finding packages in radio astronomy, and a program being developed for the Australian Square Kilometer Array Pathfinder (ASKAP) telescope. The four packages; SExtractor, SFind, IMSAD and Selavy are shown to produce source catalogues with high completeness and reliability. In this paper we analyse the small fraction (~1%) of cases in which these packages do not perform well. This small fraction of sources will be of concern for the next generation of radio surveys which will produce many thousands of sources on a daily basis, in particular for blind radio transients surveys. From our analysis we identify the ways in which the underlying source finding algorithms fail. We demonstrate a new source finding algorithm Aegean, based on the application of a Laplacian kernel, which can avoid these problems and can produce complete and reliable source catalogues for the next generation of radio surveys.

astro-ph.IM↗

arXiv2012

VAST - a real-time pipeline for detecting radio transients and variables on the Australian SKA Pathfinder (ASKAP) telescope

The Australian SKA Pathfinder (ASKAP) is a next generation radio telescope currently under construction in Western Australia. The fast survey speed and wide field of view make it an ideal instrument for blind transients searches. The ASKAP Variable and Slow Transients (VAST) survey is a one of the major science programs planned for ASKAP. The scientific goals of VAST include the detection and characterisation of a wide range of transient and variable phenomena, from gamma-ray burst afterglows to extreme scattering events, on timescales of 5 seconds or longer. We describe the data and processing challenges involved in running the VAST real-time transient detection pipeline. ASKAP will produce 2.5 GB of visibility data per second, transformed into one 8GB image cube every 5 seconds. Each cube will contain approximately twenty 100 megapixel images with 100s of radio sources detected in each epoch. The VAST pipeline will measure and monitor all of these sources, detect variables and transients and generate alerts using the VOEvent framework. The goal of the VAST Design Study is to develop a prototype pipeline to establish and demonstrate the functionality of the final ASKAP pipeline. We give an overview of the prototype pipeline's functionality, technical implementation and current status.

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arXiv2012

Using the VO to Study the Time Domain

Just as the astronomical "Time Domain" is a catch-phrase for a diverse group of different science objectives involving time-varying phenomena in all astrophysical regimes from the solar system to cosmological scales, so the "Virtual Observatory" is a complex set of community-wide activities from archives to astroinformatics. This workshop touched on some aspects of adapting and developing those semantic and network technologies in order to address transient and time-domain research challenges. It discussed the VOEvent format for representing alerts and reports on celestial transient events, the SkyAlert and ATELstream facilities for distributing these alerts, and the IVOA time-series protocol and time-series tools provided by the VAO. Those tools and infrastructure are available today to address the real-world needs of astronomers.

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arXiv2011

The completeness and reliability of threshold and false-discovery-rate source extraction algorithms for compact continuum sources

The process of determining the number and characteristics of sources in astronomical images is so fundamental to a large range of astronomical problems that it is perhaps surprising that no standard procedure has ever been defined that has well understood properties with a high degree of statistical rigour on completeness and reliability. There are now a large number of commonly used software tools for accomplishing this task, typically with different tools being used for images acquired using different technologies. Despite this, there have been relatively few quantitative analyses of the robustness or reliability of individual tools, or the details of the techniques they implement. We have an opportunity to redress this omission in the context of surveys planned with the Australian Square Kilometre Array Pathfinder (ASKAP). The Evolutionary Map of the Universe (EMU) survey with ASKAP, a continuum survey of the Southern Hemisphere up to declination +30 deg, aims to utilise an automated source identification and measurement approach that is demonstrably optimal, to maximise the reliability, utility and robustness of the resulting radio source catalogues. A key stage in source extraction methods is the background estimation (background level and noise level) and the choice of a threshold high enough to reject false sources yet not so high that the catalogues are significantly incomplete. In this analysis we present results from testing such algorithms as implemented in the SExtractor, Selavy (Duchamp), and sfind tools on simulated data. In particular the effects of background estimation, threshold and false-discovery rate settings are explored. For parameters that give similar completeness, the false-discovery rate method employed by sfind results in a more reliable catalogue compared to the peak threshold methods of SExtractor and Selavy.

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arXiv2011

The Australia Telescope 20GHz Survey: Hardware, Observing Strategy, and Scanning Survey Catalog

The Australia Telescope 20GHz (AT20G) survey is a large area (2π sr), sensitive (40mJy), high frequency (20GHz) survey of the southern sky. The survey was conducted in two parts: an initial fast scanning survey, and a series of more accurate follow-up observations. The follow-up survey catalog has been presented by Murphy et al. 2010. In this paper we discuss the hardware setup and scanning survey strategy as well as the production of the scanning survey catalog.

astro-ph.IM↗

arXiv2011

Optical Properties of High-Frequency Radio Sources from the Australia Telescope 20 GHz (AT20G) Survey

Our current understanding of radio-loud AGN comes predominantly from studies at frequencies of 5 GHz and below. With the recent completion of the Australia Telescope 20 GHz (AT20G) survey, we can now gain insight into the high-frequency radio properties of AGN. This paper presents supplementary information on the AT20G sources in the form of optical counterparts and redshifts. Optical counterparts were identified using the SuperCOSMOS database and redshifts were found from either the 6dF Galaxy survey or the literature. We also report 144 new redshifts. For AT20G sources outside the Galactic plane, 78.5% have optical identifications and 30.9% have redshift information. The optical identification rate also increases with increasing flux density. Targets which had optical spectra available were examined to obtain a spectral classification. There appear to be two distinct AT20G populations; the high luminosity quasars that are generally associated with point-source optical counterparts and exhibit strong emission lines in the optical spectrum, and the lower luminosity radio galaxies that are generally associated with passive galaxies in both the optical images and spectroscopic properties. It is suggested that these different populations can be associated with different accretion modes (cold-mode or hot-mode). We find that the cold-mode sources have a steeper spectral index and produce more luminous radio lobes, but generally reside in smaller host galaxies than their hot-mode counterparts. This can be attributed to the fact that they are accreting material more efficiently. Lastly, we compare the AT20G survey with the S-cubed semi-empirical (S3-SEX) models and conclude that the S3-SEX models need refining to correctly model the compact cores of AGN. The AT20G survey provides the ideal sample to do this.

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arXiv2011

The radio continuum structure of Centaurus A at 1.4 GHz

A 45 deg2 radio continuum imaging campaign of the nearest radio galaxy, Centaurus A, is reported. Using the Australia Telescope Compact Array and the Parkes 64m radio telescope at 1.4 GHz, the spatial resolution of the resultant image is ~600 pc, resolving the ~500 kpc giant radio lobes with approximately five times better physical resolution compared to any previous image, and making this the most detailed radio continuum image of any radio galaxy to date. In this paper, we present these new data and discuss briefly some of the most interesting morphological features that we have discovered in the images... abstract truncated.

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arXiv2011

The LBA Calibrator Survey of southern compact extragalactic radio sources - LCS1

We present a catalogue of positions and correlated flux densities of 410 flat-spectrum, compact extragalactic radio sources, previously detected in the AT20G survey. The catalogue spans the declination range -90deg, -40deg and was constructed from four 24 hour VLBI observing sessions with the Australian Long Baseline Array made at 8.3 GHz. The detection rate in these experiments is 97%. The median uncertainty of source positions is 2.6 mas, the median correlated flux density at baseline projections lengths longer than 1000 km is 0.14 Jy. The goal of this work is 1) to provide a pool of sources with positions known at the milliarcsecond level of accuracy that are needed for phase referencing observations, for geodetic VLBI, and for space navigation; 2) to extend the complete flux-limited sample of compact extragalactic sources to the southern hemisphere; and 3) to investigate parsec-scale properties of high-frequency selected sources from the AT20G survey. As a result of the campaign, the number of compact radio sources with declinations < -40deg detectable with VLBI with measured correlated flux densities and positions known with the milliarcsec level of accuracies increased by a factor of 3.5. The catalogue and supporting material is available at http://astrogeo.org/lcs1 .

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arXiv2011

Visibility stacking in the quest for SNIa radio emission

We describe the process of stacking radio interferometry visibilities to form a deep composite image and its application to the observation of transient phenomena. We apply "visibility stacking" to 46 archival Very Large Array observations of nearby type Ia supernovae (SNeIa). This new approach provides an upper limit on the SNIa ensemble peak radio luminosity of 1.2x10^{25}erg/s/Hz at 5GHz, which is 5-10 times lower than previously measured. This luminosity implies an upper limit on the average companion stellar wind mass loss rate of 1.3x10^{-7}M_o/yr. This mass loss rate is consistent with the double degenerate scenario for SNeIa and rules out intermediate and high mass companions in the single degenerate scenario. In the era of time domain astronomy, techniques such as visibility stacking will be important in extracting the maximum amount of information from observations of populations of short lived events.

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arXiv2010

Multi-wavelength diagnostic properties of Galactic Planetary Nebulae detected by GLIMPSE-I

We uniformly analyze 136 optically detected PNe and candidates from the GLIMPSE-I survey in order to to develop robust, multi-wavelength, classification criteria to augment existing diagnostics and provide pure PN samples. PNe represent powerful astrophysical probes. They are important dynamical tracers, key sources of ISM chemical enrichment, windows into late stellar evolution, and potent cosmological yardsticks. But their utility depends on separating them unequivocally from the many nebular mimics which can strongly resemble bona fide PNe in traditional optical images and spectra. We merge new PNe from the carefully evaluated, homogeneous MASH-I and MASH-II surveys, which offer a wider evolutionary range of PNe than hitherto available, with previously known PNe classified by SIMBAD. Mid-infrared (MIR) measurements vitally complement optical data because they reveal other physical processes and morphologies via fine-structure lines, molecular bands and dust. MIR colour-colour planes, optical emission line ratios and radio fluxes show the unambiguous classification of PNe to be complex, requiring all available evidence. Statistical trends provide predictive value and we offer quantitative MIR criteria to determine whether an emission nebula is most likely to be a PN or one of the frequent contaminants such as compact HII regions or symbiotic systems. Prerequisites have been optical images and spectra but MIR morphology, colours, environment and a candidate's MIR/radio flux ratio provide a more rigorous classification. Our ultimate goal is to recognize PNe using only MIR and radio characteristics, enabling us to trawl for PNe effectively even in heavily obscured regions of the Galaxy.

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