Browsing by Author "Jauncey, D. L."
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Item DUAL-FREQUENCY OBSERVATIONS OF 140 COMPACT, FLAT-SPECTRUM ACTIVE GALACTIC NUCLEI FOR SCINTILLATION-INDUCED VARIABILITY(IOP, 2011-08-30) Koay, J. Y.; Macquart, J.-P.; Rickett, B. J.; Bignall, H. E.; Lovell, J. E. J.; Reynolds, C.; Jauncey, D. L.; Pursimo, T.; Kedziora-Chudczer, L.; Ojha, R.The 4.9 GHz Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey detected a drop in interstellar scintillation (ISS) for sources at redshifts z gsim 2, indicating an apparent increase in angular diameter or a decrease in flux density of the most compact components of these sources relative to their extended emission. This can result from intrinsic source size effects or scatter broadening in the intergalactic medium (IGM) in excess of the expected (1 + z)¹∕ ² angular diameter scaling of brightness temperature limited sources resulting from cosmological expansion. We report here 4.9 GHz and 8.4 GHz observations and data analysis for a sample of 140 compact, flat-spectrum sources which may allow us to determine the origin of this angular diameter-redshift relation by exploiting their different wavelength dependences. In addition to using ISS as a cosmological probe, the observations provide additional insight into source morphologies and the characteristics of ISS. As in the MASIV Survey, the variability of the sources is found to be significantly correlated with line-of-sight Hα intensities, confirming its link with ISS. For 25 sources, time delays of about 0.15-3 days are observed between the scintillation patterns at both frequencies, interpreted as being caused by a shift in core positions when probed at different optical depths. Significant correlation is found between ISS amplitudes and source spectral index; in particular, a large drop in ISS amplitudes is observed at α < –0.4 confirming that steep spectrum sources scintillate less. We detect a weakened redshift dependence of ISS at 8.4 GHz over that at 4.9 GHz, with the mean variance at four-day timescales reduced by a factor of 1.8 in the z > 2 sources relative to the z < 2 sources, as opposed to the factor of three decrease observed at 4.9 GHz. This suggests scatter broadening in the IGM, but the interpretation is complicated by subtle selection effects that will be explored further in a follow-up paper.Item The Knotty Question of the Jet of PKS B1421–490(IOP, 2005-10-04) Gelbord, J. M.; Marshall, H. L.; Worrall, D. M.; Birkinshaw, M.; Lovell, J. E. J.; Ojha, R.; Godfrey, L.; Schwartz, D. A.; Perlman, E. S.; Georganopoulos, M.; Murphy, D. W.; Jauncey, D. L.We report the discovery of unusually strong optical and X-ray emission associated with a knot in the radio jet of PKS B1421-490. The knot is the brightest feature observed beyond the radio band, with knot/core flux ratios ~300 and 3.7 at optical and X-ray frequencies, respectively. We interpret the extreme optical output of the knot as synchrotron emission. The nature of the X-ray emission is unclear. We consider a second synchrotron component, inverse Compton emission from a relativistic, decelerating jet, and the possibility that this feature is a chance superposition of an unusual BL Lac object.Item The Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey. II. The First Four Epochs(IOP, 2008-12-10) Lovell, J. E. J.; Rickett, B. J.; Macquart, J-P; Jauncey, D. L.; Bignall, H. E.; Kedziora-Chudczer, L.; Ojha, R.; Pursimo, T.; Dutka, M.; Senkbeil, C.; Shabala, S.We report on the variability of 443 flat-spectrum, compact radio sources monitored using the VLA for 3 days in four epochs at ~4 month intervals at 5 GHz as part of the Micro-Arcsecond Scintillation-Induced Variability (MASIV) survey. Over half of these sources exhibited 2%-10% rms variations on timescales over 2 days. We analyzed the variations by two independent methods and find that the rms variability amplitudes of the sources correlate with the emission measure in the ionized interstellar medium along their respective lines of sight. We thus link the variations with interstellar scintillation of components of these sources, with some (unknown) fraction of the total flux density contained within a compact region of angular diameter in the range 10-50 μas. We also find that the variations decrease for high mean flux density sources and, most importantly, for high-redshift sources. The decrease in variability is probably due either to an increase in the apparent diameter of the source or to a decrease in the flux density of the compact fraction beyond z ~ 2. Here we present a statistical analysis of these results, and a future paper will discuss the cosmological implications in detail.Item Milliarcsecond Observations of Microarcsecond AGN(Astronomical Society of the Pacific Conference Series, 2008) Lazio, J.; Ojha, R.; Fey, A. L.; Kedziora-Chudczer, L.; Cordes, J. M.; Jauncey, D. L.; Lovell, J. E.We report on a multi-wavelength program to examine the core structure of extremely compact AGN and to search for indications of an intergalactic medium. We have conducted Very Long Baseline Array (VLBA) observations of 58 compact extragalactic sources. Approximately 75% of these objects display interstellar scintillation, in the form of intraday variability, indicative of extremely compact structure (∼ 10 μarcsec). We confirm earlier, single-wavelength observations that found scintillating sources to be more core-dominated than non-scintillating sources [Ojha et al. (2004: ApJ, 614, 607)], consistent with expectations from their scintillation. However, scintillating and non-scintillating sources display comparable levels of radio-wave scattering (∼ 2 mas at 1 GHz). We also have redshifts for 63% of the sources and find no trend of scattering strength with redshift. We use these observations to place limits on the level of turbulence that the ionized intergalactic medium can support. Work in progress includes expanding the number of sources for which redshifts have been measured. The VLBA is a facility instrument of the National Radio Astronomy Observatory, which is operated by Associated Universities, Inc. under contract from the National Science Foundation. Basic research in radio astronomy at the NRL is supported by 6.1 Base Funding.Item Radio and gamma-ray properties of extragalactic jets from the TANAMI sample(EDP sciences, 2016-05-04) Böck, M.; Kadler, M.; Müller, C.; Tosti, G.; Bastieri, D.; Burnett, T.; Carpenter, B.; Cavazzuti, E.; Dutka, M.; Blanchard, J.; Edwards, P. G.; Hase, H.; Horiuchi, S.; Jauncey, D. L.; Krauß, F.; Lister, M. L.; Lovell, J. E. J.; Lott, B.; Murphy, D. W.; Phillips, C.; Plötz, C.; Pursimo, T.; Quick, J.; Ros, E.; Taylor, G.; Thompson, D. J.; Tingay, S. J.; Tzioumis, A.; Zensus, J. A.The TANAMI program has been observing parsec-scale radio jets of southern (declination south of − 30°) γ-ray bright AGN, simultaneously with Fermi/LAT monitoring of their γ-ray emission, via high-resolution radio imaging with Very Long Baseline Interferometry techniques. We present the radio and γ-rayproperties of the TANAMI sources based on one year of contemporaneous TANAMI and Fermi/LAT data. A large fraction (72%) of the TANAMI sample can be associated with bright γ-ray sources for this time range. Association rates differ for different optical classes with all BL Lacs, 76% of quasars, and just 17% of galaxies detected by the LAT. Upper limits were established on the γ-ray flux from TANAMI sources not detected by LAT. This analysis led to the identification of three new Fermi sources whose detection was later confirmed. The γ-ray and radio luminosities are related by Lγ ∝ Lᵣ⁰·⁸⁹±⁰·⁰⁴. The brightness temperatures of the radio cores increase with the average γ-ray luminosity and the presence of brightness temperatures above the inverse Compton limit implies strong Doppler boosting in those sources. The undetected sources have lower γ/radio luminosity ratios and lower contemporaneous brightness temperatures. Unless the Fermi/LAT-undetected blazars are much γ-ray-fainter than the Fermi/LAT-detected sources, their γ-ray luminosity should not be significantly lower than the upper limits calculated here.Item TANAMI: tracking active galactic nuclei with austral milliarcsecond interferometry(EDP sciences, 2010-09-10) Ojha, R.; Kadler, M.; Böck, M.; Booth, R.; Dutka, M. S.; Edwards, P. G.; Fey, A. L.; Fuhrmann, L.; Gaume, R. A.; Hase, H.; Horiuchi, S.; Jauncey, D. L.; Johnston, K. J.; Katz, U.; Lister, M.; Lovell, J. E. J.; Müller, C.; Plötz, C.; Quick, J. F. H.; Ros, E.; Taylor, G. B.; Thompson, D. J.; Tingay, S. J.; Tosti, G.; Tzioumis, A. K.; Wilms, J.; Zensus, J. A.Context. A number of theoretical models vie to explain the γ-ray emission from active galactic nuclei (AGN). This was a key discovery of EGRET. With its broader energy coverage, higher resolution, wider field of view and greater sensitivity, the Large Area Telescope (LAT) of the Fermi Gamma-ray Space Telescope is dramatically increasing our knowledge of AGN γ-ray emission. However, discriminating between competing theoretical models requires quasi-simultaneous observations across the electromagnetic spectrum. By resolving the powerful parsec-scale relativistic outflows in extragalactic jets and thereby allowing us to measure critical physical properties, Very Long Baseline Interferometry observations are crucial to understanding the physics of extragalactic γ-ray objects. Aims. We introduce the TANAMI program (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry) which is monitoring an initial sample of 43 extragalactic jets located south of -30 degrees declination at 8.4 GHz and 22 GHz since 2007. All aspects of the program are discussed. First epoch results at 8.4 GHz are presented along with physical parameters derived therefrom. Methods. These observations were made during 2007/2008 using the telescopes of the Australian Long Baseline Array in conjunction with Hartebeesthoek in South Africa. These data were correlated at the Swinburne University correlator. Results. We present first epoch images for 43 sources, some observed for the first time at milliarcsecond resolution. Parameters of these images as well as physical parameters derived from them are also presented and discussed. These and subsequent images from the TANAMI survey are available at http://pulsar.sternwarte.uni-erlangen.de/tanami/. Conclusions. We obtain reliable, high dynamic range images of the southern hemisphere AGN. All the quasars and BL Lac objects in the sample have a single-sided radio morphology. Galaxies are either double-sided, single-sided or irregular. About 28% of the TANAMI sample has been detected by LAT during its first three months of operations. Initial analysis suggests that when galaxies are excluded, sources detected by LAT have larger opening angles than those not detected by LAT. Brightness temperatures of LAT detections and non-detections seem to have similar distributions. The redshift distributions of the TANAMI sample and sub-samples are similar to those seen for the bright γ-ray AGN seen by LAT and EGRET but none of the sources with a redshift above 1.8 have been detected by LAT.Item WHY DO COMPACT ACTIVE GALACTIC NUCLEI AT HIGH REDSHIFT SCINTILLATE LESS?(IOP, 2012-08-12) Koay, J. Y.; Macquart, J-P.; Rickett, B. J.; Bignall, H. E.; Jauncey, D. L.; Pursimo, T.; Reynolds, C.; Lovell, J. E. J.; Kedziora-Chudczer, L.; Ojha, R.The fraction of compact active galactic nuclei (AGNs) that exhibit interstellar scintillation (ISS) at radio wavelengths, as well as their scintillation amplitudes, have been found to decrease significantly for sources at redshifts z >~ 2. This can be attributed to an increase in the angular sizes of the μas-scale cores or a decrease in the flux densities of the compact μas cores relative to that of the mas-scale components with increasing redshift, possibly arising from (1) the space-time curvature of an expanding universe, (2) AGN evolution, (3) source selection biases, (4) scatter broadening in the ionized intergalactic medium (IGM) and intervening galaxies, or (5) gravitational lensing. We examine the frequency scaling of this redshift dependence of ISS to determine its origin, using data from a dual-frequency survey of ISS of 128 sources at 0 <~ z <~ 4. We present a novel method of analysis which accounts for selection effects in the source sample. We determine that the redshift dependence of ISS is partially linked to the steepening of source spectral indices (α⁸·⁴ ₄.₉) with redshift, caused either by selection biases or AGN evolution, coupled with weaker ISS in the α⁸·⁴ ₄·₉ < –0.4 sources. Selecting only the –0.4 < α⁸·⁴ ₄.₉ < 0.4 sources, we find that the redshift dependence of ISS is still significant, but is not significantly steeper than the expected (1 + z)⁰·⁵ scaling of source angular sizes due to cosmological expansion for a brightness temperature and flux-limited sample of sources. We find no significant evidence for scatter broadening in the IGM, ruling it out as the main cause of the redshift dependence of ISS. We obtain an upper limit to IGM scatter broadening of <~ 110 μas at 4.9 GHz with 99% confidence for all lines of sight and as low as <~ 8 μas for sight lines to the most compact, ~10 μas sources.