arXiv subjects

S. J. Tingay

Publications and source records attributed to S. J. Tingay.

At least 181 records · Page 10Linked to original sources

arXiv2007

DiFX: A software correlator for very long baseline interferometry using multi-processor computing environments

We describe the development of an FX style correlator for Very Long Baseline Interferometry (VLBI), implemented in software and intended to run in multi-processor computing environments, such as large clusters of commodity machines (Beowulf clusters) or computers specifically designed for high performance computing, such as multi-processor shared-memory machines. We outline the scientific and practical benefits for VLBI correlation, these chiefly being due to the inherent flexibility of software and the fact that the highly parallel and scalable nature of the correlation task is well suited to a multi-processor computing environment. We suggest scientific applications where such an approach to VLBI correlation is most suited and will give the best returns. We report detailed results from the Distributed FX (DiFX) software correlator, running on the Swinburne supercomputer (a Beowulf cluster of approximately 300 commodity processors), including measures of the performance of the system. For example, to correlate all Stokes products for a 10 antenna array, with an aggregate bandwidth of 64 MHz per station and using typical time and frequency resolution presently requires of order 100 desktop-class compute nodes. Due to the effect of Moore's Law on commodity computing performance, the total number and cost of compute nodes required to meet a given correlation task continues to decrease rapidly with time. We show detailed comparisons between DiFX and two existing hardware-based correlators: the Australian Long Baseline Array (LBA) S2 correlator, and the NRAO Very Long Baseline Array (VLBA) correlator. In both cases, excellent agreement was found between the correlators. Finally, we describe plans for the future operation of DiFX on the Swinburne supercomputer, for both astrophysical and geodetic science.

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arXiv2006

The pc-scale structure and evolution of BL Lac PKS 0003-066

We present the results of global VLBI observations of the BL Lac object PKS 0003-066 at 2.3 and 8.6 GHz, over a 10 year period using data from new and archived VLBA, geodetic VLBI, and global VLBI (including the use of new disk-based recording and software correlation systems) observations. Inter-continental baselines resolve this source into a compact core and three jet components that move away from the core with proper motions ranging between \~0.1 mas/yr for the outer jet component and ~1.4 mas/yr for the fastest inner jet component, i.e. apparent speeds of ~2.0 c to ~21.6 c respectively. This result is in contrast to previous studies of this source which did not reveal significant jet motions from limited data sets. We find that the fast inner jet components catch up to the more slowly moving outer jet components and interact strongly, causing local brightening of the jet in the interaction. We document the body of VLBI data for this source, as well as other supporting radio observations, as a basis for future work that will investigate the underlying physics of the pc-scale jet in PKS 0003-066.

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arXiv2004

The Halo, Hot Spots and Jet/Cloud Interaction of PKS 2153--69

We report Chandra X-ray Observatory and 1.4 GHz Australian Long Baseline Array (LBA) observations of the radio galaxy PKS 2153--69 and its environment. The Chandra image reveals a roughly spherical halo of hot gas extending out to 30 kpc around PKS 2153--69. Two depressions in the surface brightness of the X-ray halo correspond to the large scale radio lobes, and interpreting these as cavities inflated with radio plasma we infer a jet power of 4x10^42 erg/s. Both radio lobes contain hot spots that are detected by Chandra. In addition, the southern hot spot is detected in the 1.4 GHz LBA observation, providing the highest linear resolution image of a radio lobe hot spot to date. The northern hot spot was not detected in the LBA observation. The radio to X-ray spectra of the hot spots are consistent with a simple power law emission model. The nucleus has an X-ray spectrum typical of a type 1 active galactic nucleus, and the LBA observation shows a one-sided nuclear jet on 0.1" scales. Approximately 10" northeast of the nucleus, X-ray emission is associated with an extra-nuclear cloud. The X-ray emission from the cloud can be divided into two regions, an unresolved western component associated with a knot of radio emission (in a low resolution map), and a spatially extended eastern component aligned with the pc-scale jet and associated with highly ionized optical line-emitting clouds. The X-ray spectrum of the eastern component is very soft (Gamma > 4.0 for a power law model or kT ~0.22 keV for a thermal plasma). The LBA observation did not detect compact radio emission from the extra-nuclear cloud. (Abstract truncated).

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arXiv2004

The 2003 radio outburst of a new X-ray transient: XTE J1720-318

We present radio observations of the black hole X-ray transient XTE J1720-318, which was discovered in 2003 January as it entered an outburst. We analyse the radio data in the context of the X-ray outburst and the broad-band spectrum. An unresolved radio source was detected during the rising phase, reaching a peak of nearly 5 mJy approximately coincident with the peak of the X-ray lightcurve. Study of the spectral indices suggests that at least two ejection events took place, the radio-emitting material expanding and becoming optically thin as it faded. The broad-band spectra suggested that the accretion disc dominated the emission, as expected for a source in the high/soft state. The radio emission decayed to below the sensitivity of the telescopes for ~6 weeks but switched on again during the transition of the X-ray source to the low/hard state. At least one ``glitch'' was superimposed on the otherwise exponential decay of the X-ray lightcurve, which was reminiscent of the multiple jet ejections of XTE J1859+226. We also present a K_s-band image of XTE J1720-318 and its surrounding field taken with the VLT.

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arXiv2004

The VSOP 5 GHz AGN Survey: III. Imaging Results for the First 102 Sources

The VSOP mission is a Japanese-led project to study radio sources with sub-milliarcsec resolution using an orbiting 8 m telescope, HALCA, along with global arrays of Earth-based telescopes. Approximately 25% of the observing time is devoted to a survey of compact AGN which are stronger than 1 Jy at 5 GHz-the VSOP AGN Survey. This paper, the third in the series, presents the results from the analysis of the first 102 Survey sources. We present high resolution images and plots of visibility amplitude versus projected baseline length. In addition, model-fit parameters to the primary radio components are listed, and from these the angular size and brightness temperature for the radio cores are calculated. For those sources for which we were able to determine the source frame core brightness temperature, a significant fraction (53 out of 98) have a source frame core brightness temperature in excess of 10^12 K. The maximum source frame core brightness temperature we observed was 1.2 X 10^13 K. Explaining a brightness temperature this high requires an extreme amount of relativistic Doppler beaming. Since the maximum brightness temperature one is able to determine using only ground-based arrays is of the order of 10^12 K, our results confirm the necessity of using space VLBI to explore the extremely high brightness temperature regime.

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arXiv2004

New candidate GHz Peaked Spectrum and Compact Steep Spectrum Sources

Data from a recent Australia Telescope Compact Array (ATCA) program of multi-frequency, multi-epoch monitoring of 202 active galactic nuclei with declinations < +10 degrees have been searched for GHz Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) radio sources. Supplementary data at higher and lower frequencies, where available, have been used to further examine the spectral properties of previously reported and new candidate GPS and CSS sources. The ATCA monitoring program has allowed the variability and polarization properties of sources previously reported as GPS and CSS sources, and the majority of new GPS and CSS candidates, to be investigated, confirming that these are useful diagnostics in discriminating genuine GPS and CSS sources from variable sources that display similar spectra only temporarily. GPS sources are confirmed to be generally more compact, and less polarized, than CSS sources, although CSS sources show evidence for being somewhat less variable than GPS sources at 1.4 and 2.5GHz. In addition, the widths of GPS spectra are examined, and a significant difference is found in the GPS sample of Snellen et al. (2000) between sources with compact double (CD) or compact symmetric object (CSO) morphologies and sources with other morphologies, in that CD and CSO sources have generally narrower spectra. Possible reasons for this difference are considered.

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arXiv2004

The VSOP 5 GHz AGN Survey II. Data Calibration and Imaging

The VSOP mission is a Japanese-led project to study radio sources with sub-milliarcsecond angular resolution using an orbiting 8-m telescope, HALCA and global arrays of Earth-based telescopes. Approximately 25% of the observing time has been devoted to a survey of compact AGN at 5 GHz which are stronger than 1 Jy -- the VSOP AGN Survey. This paper, the second in a series, describes the data calibration, source detection, self-calibration, imaging and modeling, and gives examples illustrating the problems specific to space VLBI. The VSOP Survey web-site which contains all results and calibrated data is described.

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arXiv2002

Rapid Variability and Annual Cycles in the Characteristic Time-scale of the Scintillating Source PKS 1257-326

Rapid radio intra-day variability (IDV) has been discovered in the southern quasar PKS 1257-326. Flux density changes of up to 40% in as little as 45 minutes have been observed in this source, making it, along with PKS 0405-385 and J1819+3845, one of the three most rapid IDV sources known. We have monitored the IDV in this source with the Australia Telescope Compact Array (ATCA) at 4.8 and 8.6 GHz over the course of the last year, and find a clear annual cycle in the characteristic time-scale of variability. This annual cycle demonstrates unequivocally that interstellar scintillation is the cause of the rapid IDV at radio wavelengths observed in this source. We use the observed annual cycle to constrain the velocity of the scattering material, and the angular size of the scintillating component of PKS 1257-326. We observe a time delay, which also shows an annual cycle, between the similar variability patterns at the two frequencies. We suggest that this is caused by a small (~10 microarcsecond) offset between the centroids of the 4.8 and 8.6 GHz components, and may be due to opacity effects in the source. The statistical properties of the observed scintillation thus enable us to resolve source structure on a scale of ~10 microarcseconds, resolution orders of magnitude higher than current VLBI techniques allow. General implications of scintillation for the physical properties of sources and the turbulent ISM are discussed.

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arXiv2001

The star-formation rate in the host of GRB 990712

We have observed the host galaxy of GRB 990712 at 1.4 GHz with the Australia Telescope Compact Array, to obtain an estimate of its total star-formation rate. We do not detect a source at the position of the host. The 2 sigma upper limit of 70 microJy implies that the total star-formation rate is lower than 100 Msun/yr, using conservative values for the spectral index and cosmological parameters. This upper limit is in stark contrast with recent reports of radio/submillimeter-determined star-formation rates of roughly 500 Msun/yr for two other GRB host galaxies. Our observations present the deepest radio-determined star-formation rate limit on a GRB host galaxy yet, and show that also from the unobscured radio point-of-view, not every GRB host galaxy is a vigorous starburst.

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arXiv2001

The Pearson-Readhead Survey of Compact Extragalactic Radio Sources From Space. I. The Images

We present images from a space-VLBI survey using the facilities of the VLBI Space Observatory Programme (VSOP), drawing our sample from the well-studied Pearson-Readhead survey of extragalactic radio sources. Our survey has taken advantage of long space-VLBI baselines and large arrays of ground antennas, such as the Very Long Baseline Array and European VLBI Network, to obtain high resolution images of 27 active galactic nuclei, and to measure the core brightness temperatures of these sources more accurately than is possible from the ground. A detailed analysis of the source properties is given in accompanying papers. We have also performed an extensive series of simulations to investigate the errors in VSOP images caused by the relatively large holes in the (u,v) plane when sources are observed near the orbit normal direction. We find that while the nominal dynamic range (defined as the ratio of map peak to off-source error) often exceeds 1000:1, the true dynamic range (map peak to on-source error) is only about 30:1 for relatively complex core-jet sources. For sources dominated by a strong point source, this value rises to approximately 100:1. We find the true dynamic range to be a relatively weak function of the difference in position angle (PA) between the jet PA and (u,v) coverage major axis PA. For low signal-to-noise regions typically located down the jet away from the core, large errors can occur, causing spurious features in VSOP images that should be interpreted with caution.

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arXiv2001

The Pearson-Readhead Survey of Compact Extragalactic Radio Sources From Space. II. Analysis of Source Properties

We have performed a multi-dimensional correlation analysis on the observed properties of a statistically complete core-selected sample of compact radio-loud active galactic nuclei, based on data from the VLBI Space Observing Programme (Paper I) and previously published studies. Our sample is drawn from the well-studied Pearson-Readhead (PR) survey, and is ideally suited for investigating the general effects of relativistic beaming in compact radio sources. In addition to confirming many previously known correlations, we have discovered several new trends that lend additional support to the beaming model. These trends suggest that the most highly beamed sources in core-selected samples tend to have a) high optical polarizations; b) large pc/kpc-scale jet misalignments; c) prominent VLBI core components; d) one-sided, core, or halo radio morphology on kiloparsec scales; e) narrow emission line equivalent widths; and f) a strong tendency for intraday variability at radio wavelengths. We have used higher resolution space and ground-based VLBI maps to confirm the bi-modality of the jet misalignment distribution for the PR survey, and find that the sources with aligned parsec- and kiloparsec-scale jets generally have arcsecond-scale radio emission on both sides of the core. The aligned sources also have broader emission line widths. We find evidence that the BL Lacertae objects in the PR survey are all highly beamed, and have very similar properties to the high-optically polarized quasars, with the exception of smaller redshifts. A cluster analysis on our data shows that after partialing out the effects of redshift, the luminosities of our sample objects in various wave bands are generally well-correlated with each other, but not with other source properties.

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arXiv2001

Measuring the brightness temperature distribution of extragalactic radio sources with space VLBI

We have used VSOP space very long baseline interferometry observations to measure the brightness temperature distribution of a well-defined sub-set of the Pearson-Readhead sample of extragalactic radio sources. VLBI which is restricted to Earth-diameter baselines is not generally sensitive to emitting regions with brightness temperatures greater than approximately $10^{12}$ K, coincidentally close to theoretical estimates of brightness temperature limits, $10^{11} - 10^{12}$ K. We find that a significant proportion of our sample have brightness temperatures greater than $10^{12}$ K; many have unresolved components on the longest baselines, and some remain completely unresolved. These observations begin to bridge the gap between the extended jets seen with ground-based VLBI and the microarcsecond structures inferred from intraday variability, evidenced here by the discovery of a relationship between intraday variability and VSOP-measured brightness temperature, likely due to the effects of relativistic beaming. Also, lower limits on jet Lorentz factors, estimated from space VLBI observations, are starting to challenge numerical simulations that predict low Lorentz factor jets.

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arXiv2000

The 1997 hard state outburst of the X-ray transient GS 1354-64 / BW Cir

We present observations of the 1997 outburst of the X-ray transient GS 1354-64 (BW Cir) at X-ray, optical and, for the first time, radio wavelengths; this includes upper limits to the linear and circular polarisation of the radio data. The X-ray outburst was unusual in that the source remained in the low/hard X-ray state throughout; the X-ray peak was also preceded by at least one optical outburst, suggesting that it was an `outside-in' outburst - similar to those observed in dwarf novae systems, although possibly taking place on a viscous timescale in this case. It therefore indicates that the optical emission was not dominated by the reprocessing of X-rays but that instead we see the instability directly. While the radio source was too faint to detect any extended structure, spectral analysis of the radio data and a comparison with other similar systems suggests that mass ejections, probably in the form of a jet, took place and that the emitted synchrotron spectrum may have extended as far as infrared wavelengths. Finally, we compare this 1997 outburst of GS 1354-64 with possible previous outbursts and also with other hard state objects, both transient and persistent. It appears that a set of characteristics -- such as a weak, flat spectrum radio jet, a mHz QPO increasing in frequency, a surprisingly high optical:X-ray luminosity ratio and the observed optical peak preceding the X-ray peak -- may be common to all hard state X-ray transients.

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arXiv2000

Chandra Discovery of a 100 kpc X-ray Jet in PKS 0637--752

The quasar PKS 0637-753, the first celestial X-ray target of the Chandra X-ray Observatory, has revealed asymmetric X-ray structure extending from 3 to 12 arcsec west of the quasar, coincident with the inner portion of the jet previously detected in a 4.8 GHz radio image (Tingay et al. 1998). At a redshift of z=0.651, the jet is the largest (~100 kpc) and most luminous (~10^{44.6} ergs/s) of the few so far detected in X-rays. This letter presents a high resolution X-ray image of the jet, from 42 ks of data when PKS 0637-753 was on-axis and ACIS-S was near the optimum focus. For the inner portion of the radio jet, the X-ray morphology closely matches that of new ATCA radio images at 4.8 and 8.6 GHz. Observations of the parsec scale core using the VSOP space VLBI mission show structure aligned with the X-ray jet, placing important constraints on the X-ray source models. HST images show that there are three small knots coincident with the peak radio and X-ray emission. Two of these are resolved, which we use to estimate the sizes of the X-ray and radio knots. The outer portion of the radio jet, and a radio component to the east, show no X-ray emission to a limit of about 100 times lower flux. The X-ray emission is difficult to explain with models that successfully account for extra-nuclear X-ray/radio structures in other active galaxies. We think the most plausible is a synchrotron self-Compton (SSC) model, but this would imply extreme departures from the conventional minimum-energy and/or homogeneity assumptions. We also rule out synchrotron or thermal bremsstrahlung models for the jet X-rays, unless multicomponent or ad hoc geometries are invoked.

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arXiv2000

The Chandra X-ray Observatory Resolves the X-ray Morphology and Spectra of a Jet in PKS 0637-752

The core-dominated radio-loud quasar PKS 0637-752 (z = 0.654) was the first celestial object observed with the Chandra X-ray Observatory, offering the early surprise of the detection of a remarkable X-ray jet. Several observations with a variety of detector configurations contribute to a total exposure time with the Chandra Advanced CCD Imaging Spectrometer (ACIS; Garmire et al. 2000, in preparation) of about 100ks. A spatial analysis of all the available X-ray data, making use of Chandra's spatial resolving power of about 0.4 arcsec, reveals a jet that extends about 10 arcsec to the west of the nucleus. At least four X-ray knots are resolved along the jet, which contains about 5% of the overall X-ray luminosity of the source. Previous observations of PKS 0637-752 in the radio band (Tingay et al. 1998) had identified a kpc-scale radio jet extending to the West of the quasar. The X-ray and radio jets are similar in shape, intensity distribution, and angular structure out to about 9 arcsec, after which the X-ray brightness decreases more rapidly and the radio jet turns abruptly to the north. The X-ray luminosity of the total source is log Lx ~ 45.8 erg/s (2 - 10keV), and appears not to have changed since it was observed with ASCA in November 1996. We present the results of fitting a variety of emission models to the observed spectral distribution, comment on the non-existence of emission lines recently reported in the ASCA observations of PKS 0637-752, and briefly discuss plausible X-ray emission mechanisms.

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arXiv1999

Pictor A (PKS 0518-45) - From Nucleus to Lobes

We present radio and optical imaging and kinematic data for the radio galaxy Pictor A, including HST continuum and [OIII], emission-line images (at a resolution of 25 - 100 mas) and ground-based imaging and spectroscopy (at a resolution of ~ 1.5". The radio data include 3 cm Australia Telescope images of the core, at a resolution comparable to that of the optical, ground-based images, and a VLBI image of a jet in the compact core (at a resolution of 2 - 25 mas), which seems to align with a continuum ``jet'' found in the HST images. The core radio jet, the HST optical continuum ``jet'', and the NW H-alpha filaments all appear to point toward the optical-synchrotron hot-spot in the NW lobe of this object and are associated with a disrupted velocity field in the extended ionized gas. The ground-based spectra which cover this trajectory also yield line ratios for the ionized gas which have anomalously low [NII] (6564), suggesting either a complex, clumpy structure in the gas with a higher cloud-covering factor at larger radii and with denser clouds than is found in the nuclear regions of most NLRG and Seyfert 2 galaxies, or some other, unmodeled, mechanism for the emergent spectrum from this region. The H-alpha emission-line filaments to the N appear to be associated with a 3 cm radio continuum knot which lies in a gap in the filaments ~ 4" from the nucleus. Altogether, the data in this paper provide good circumstantial evidence for non-disruptive redirection of a radio jet by interstellar gas clouds in the host galaxy.

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arXiv1998

A 5-GHz Southern Hemisphere VLBI Survey of Compact Radio Sources - II

We report the results of a 5-GHz southern-hemisphere snapshot VLBI observation of a sample of blazars. The observations were performed with the Southern Hemisphere VLBI Network plus the Shanghai station in 1993 May. Twenty-three flat-spectrum, radio-loud sources were imaged. These are the first VLBI images for 15 of the sources. Eight of the sources are EGRET (> 100 MeV) gamma-ray sources. The milliarcsecond morphology shows a core-jet structure for 12 sources, and a single compact core for the remaining 11. No compact doubles were seen. Compared with other radio images at different epochs and/or different frequencies, 3 core-jet blazars show evidence of bent jets, and there is some evidence for superluminal motion in the cases of 2 blazars. The detailed descriptions for individual blazars are given. This is the second part of a survey: the first part was reported by Shen et al. (AJ 114(1997)1999).

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arXiv1997

A 5 GHz Southern Hemisphere VLBI Survey of Compact Radio SOurces - I

We report the results of a 5 GHz southern hemisphere VLBI survey of compact extragalactic radio sources. These observations were undertaken with the SHEVE array plus Shanghai station in November 1992. A sample of 22 sources was observed and images of 20 of them were obtained. Of the 20 sources imaged, 15 showed core-jet structure, one had a two-sided jet and 4 had only single compact cores. Eleven of the 16 core-jet (including one two-sided jet) sources show some evidence of bent jets. No compact doubles were found. A comparison with previous images and the temporal variability of the radio flux density showed evidence for superluminal motion in 4 of the sources. Five sources were high energy (>100 MeV) gamma-ray sources. Statistical analysis showed the dominance of highly polarized quasars among the detected gamma-ray sources, which emphasizes the importance of beaming effect in the gamma-ray emission.

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