Browsing by Author "Reynolds, John E."
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Item Accurate Astrometry of 22 Southern Hemisphere Radio Sources(IOP, 2003-12-03) Fey, Alan L.; Ojha, Roopesh; Jauncey, David L.; Johnston, Kenneth J.; Reynolds, John E.; Lovell, James E. J.; Tzioumis, Anastasios K.; Quick, Jonathan F. H.; Nicolson, George D.; Ellingsen, Simon P.; McCulloch, Peter M.; Koyama, YasuhiroMilliarcsecond-accurate radio positions for 22 southern hemisphere extragalactic sources are reported. These positions are derived from Mark III very long baseline interferometry observations made between 2003 February and 2003 August. The results presented here supplement an ongoing project to increase the sky density of southern hemisphere sources in order to better define the International Celestial Reference Frame and to provide additional phase-reference sources with accurate positions for use in astrophysical observations. The positions for all 22 sources are south of δ = -3° (positions for 10 of the sources are south of δ = -6° ) and represent the largest group of new milliarcsecond-accurate astrometric positions for sources in this declination range since the initial definition of the International Celestial Reference Frame. The reported positions have average formal uncertainties of 0.5 mas in right ascension and 0.6 mas in declinationItem Astrometry of 25 Southern Hemisphere Radio Sources from a VLBI Short-Baseline Survey(IOP, 2004-11) Fey, Alan L.; Ojha, Roopesh; Reynolds, John E.; Ellingsen, Simon P.; McCulloch, Peter M.; Jauncey, David L.; Johnston, Kenneth J.Milliarcsecond-accurate radio positions for 25 southern hemisphere extragalactic sources are reported. These positions are derived from Mark III Very Long Baseline Interferometry observations made between 2003 September and 2004 May on the intra-Australia baseline connecting Hobart, Tasmania and Parkes, New South Wales. The results presented here represent an ongoing program intended to increase the number of phase-reference sources with accurate positions in the southern hemisphere for use in astrophysical observations. The positions for all but 1 of the 25 sources are south of δ = -45° and have average formal uncertainties of 3 mas in α cos δ and 2 mas in δ. As the reported positions are in the frame of the International Celestial Reference Frame (ICRF), the results reported here can also be used to increase the sky density of southern hemisphere ICRF sources, although with reduced accuracy.Item Milliarcsecond-Accurate Astrometry of 34 Southern Hemisphere Radio Sources(IOP, 2006-11-01) Fey, Alan L.; Ojha, Roopesh; Quick, Jonathan F. H.; Nicolson, George D.; Lovell, James E. J.; Reynolds, John E.; Ellingsen, Simon P.; McCulloch, Peter M.; Johnston, Kenneth J.; Jauncey, David L.; Tzioumis, Anastasios K.Milliarcsecond-accurate radio positions in the International Celestial Reference Frame for 34 southern hemisphere extragalactic sources are reported. The positions are derived from Mark III/IV very long baseline interferometry observations made between 2004 August and 2006 April using radio telescopes located in Australia, South Africa, and Hawaii. Positions for 7 of the 34 sources have been reported previously by us but are reported here with significantly improved accuracy. These results supplement an ongoing project to increase the sky density of southern hemisphere sources in order to better define the International Celestial Reference Frame and to provide additional phase reference sources with accurate positions for use in astrophysical observations. Positions for all 34 sources are south of δ = -20° (positions for nine of the sources are south of δ = -60°) and have average formal uncertainties of 0.23 mas in α cos δ and 0.35 mas in δ.Item The Radio Variability of the Gravitational Lens PMN J1838−3427(IOP, 2004-12-01) Winn, Joshua N.; Lovell, James E. J.; Bignall, Hayley; Gaensler, Bryan M.; Getts, Tracy J.; Kedziora-Chudczer, Lucyna; Ojha, Roopesh; Reynolds, John E.; Tingay, Steven J.; Tzioumis, TassoWe present the results of a radio variability study of the gravitational lens PMN J1838-3427. Our motivation was to determine the Hubble constant by measuring the time delay between variations of the two quasar images. We monitored the system for 4 months (approximately 5 times longer than the expected delay) using the Australia Telescope Compact Array at 9 GHz. Although both images were variable on a timescale of a few days, no correlated intrinsic variability could be identified, and therefore no time delay could be measured. Notably, the fractional variation of the fainter image (8%) was greater than that of the brighter image (4%), whereas lensed images of a point source would have the same fractional variation. This effect can be explained, at least in part, as the refractive scintillation of both images due to the turbulent interstellar medium of the Galaxy.Item VLBA Snap-Shot Imaging of Scintillating Sources(Astronomical Society of the Pacific Conference Series, 2005) Ojha, Roopesh; Lovell, James E.; Jauncey, David L.; Kedziora-Chudczer, Lucyna; Bignall, Hayley E.; Reynolds, John E.; Tzioumis, Anastasios K.; Macquart, Jean-Pierre; Fey, Alan L.; Johnston, Kenneth J.We present VLBA snap-shot images of weak scintillating sources and compare their morphologies with those of strong scintillating sources imaged by the USNO astrometry program. It is now clear that interstellar scintillation (ISS) is the principal cause of the intra-day variability (IDV) seen in some compact, flat-spectum radio sources at centimeter wavelengths. The MASIV (Micro Arcsecond Scintillation Induced Variability) survey has been looking for new scintillating radio sources. One result from this survey is that the fraction of scintillating sources increases strongly with decreasing flux density. Here we address the possibility that the above MASIV result arises because we are seeing different mas morphologies among the weaker sources.Item VLBI Observations of Southern Hemisphere ICRF Sources. I.(IOP, 2004-06-01) Ojha, Roopesh; Fey, Alan L.; Johnston, Kenneth J.; Jauncey, David L.; Reynolds, John E.; Tzioumis, Anastasios K.; Quick, Jonathan F. H.; Nicolson, George D.; Ellingsen, Simon P.; Dodson, Richard G.; McCulloch, Peter M.We present 8.4 GHz very long baseline interferometry (VLBI) observations of 69 southern hemisphere extragalactic sources in the International Celestial Reference Frame. These are the first in a series of observations intended to image all such sources at milliarcsecond resolution in order to determine their continued suitability for reference-frame use based on intrinsic structure. We use the resultant images to calculate a core fraction, that is, the ratio of core flux density to total flux density, for all observed sources. The resulting distribution, with a mean value of 0.83, suggests that most sources are relatively compact. However, just over half the observed sources show significant extended emission in the form of multiple compact components. These sources are probably poorly suited for high-accuracy reference-frame use unless intrinsic structure and potential variability can be taken into account. Our observations represent the first large, comprehensive VLBI imaging survey in the southern hemisphere, significantly extending the existing limited VLBI surveys and, along with some well-known objects, containing many sources that have never been imaged at milliarcsecond resolution. The overlap with Very Long Baseline Array images of sources between 0° and -35° declination helps determine the limits to imaging with the southern hemisphere–accessible telescopes.Item VLBI Observations of Southern Hemisphere ICRF Sources. II. Astrometric Suitability Based on Intrinsic Structure(IOP, 2005-12-01) Ojha, Roopesh; Fey, Alan L.; Charlot, Patrick; Jauncey, David L.; Johnston, Kenneth J.; Reynolds, John E.; Tzioumis, Anastasios K.; Quick, Jonathan F. H.; Nicolson, George D.; Ellingsen, Simon P.; McCulloch, Peter M.; Koyama, YasuhiroWe present 8.4 GHz very long baseline interferometry (VLBI) observations of 48 southern hemisphere extragalactic sources from the International Celestial Reference Frame. These are the second in a series of observations intended to image all such sources in the southern hemisphere at milliarcsecond resolution and bring the total number of observed sources to 111. We use these data, together with previously published data, to quantify the magnitude of the expected effect of intrinsic source structure on astrometric bandwidth synthesis VLBI observations by calculating a "structure index" for the sources; the structure index yields an estimate of their astrometric quality. Approximately 35% of sources in our sample have a structure index indicative of compact or very compact structures. The remaining two-thirds of our sources are less compact and should probably be avoided in astrometric and geodetic VLBI experiments requiring the highest accuracy unless intrinsic source structure can be accounted for in the astrometric/geodetic analysis.Item VLBI Observations of the Gigahertz-Peaked Spectrum Galaxy PKS 1934−638(IOP, 2004-04-01) Ojha, Roopesh; Fey, Alan L.; Johnston, Kenneth J.; Tzioumis, Anastasios K.; Reynolds, John E.We present 8.4 GHz VLBI observations of the gigahertz-peaked spectrum source PKS 1934-638 made with the Australian Long Baseline Array. Our observations confirm the compact double nature of the source and yield measured separations of 42.7 ± 0.4 mas at epoch 2002 July and 42.6 ± 0.3 mas at epoch 2002 November, which, when combined with previous observations, yield a rate of separation of 23 ± 10 μas yr⁻¹. This result suggests that over a timescale of 32.1 yr, the separation of the two components has changed marginally. Nominally, this provides support to the emerging consensus that compact symmetric objects (CSOs) are young sources. Our measurement of hot spot separation has, to our knowledge, the longest temporal coverage for a CSO reported in the literature.