Search

Following our study of the radio and high-energy properties of $\gamma$-ray-emitting radio galaxies, here we investigate the kinematic and spectral properties of the parsec-scale jets of radio galaxies that have not yet been detected by Fermi-LAT. We take advantage of the regular VLBI observations provided by the TANAMI monitoring program, and explore the kinematic properties of six $\gamma$-ray-faint radio galaxies. We include publicly available VLBI kinematics of $\gamma$-ray-quiet radio galaxies monitored by the MOJAVE program and perform a Fermi-LAT analysis, deriving upper limits. We combine these results with those from our previous paper to construct the largest sample of radio galaxies with combined VLBI and $\gamma$-ray measurements to date. We find superluminal motion up to $\beta_\mathrm{app}=3.6$ in the jet of PKS 2153$-$69. We find a clear trend of higher apparent speed as a function of distance from the jet core on scales of $\sim10^5\,R_s$, corresponding to the end of the collimation and acceleration zone in nearby radio galaxies. We find evidence of subluminal apparent motion in the jets of PKS 1258$-$321 and IC 4296, and no measurable motion for PKS 1549$-$79, PKS 1733$-$565 and PKS 2027$-$308. We compare the VLBI properties of $\gamma$-ray-detected and undetected radio galaxies, and find significantly different distributions of median core flux density, and, possibly, of median core brightness temperature. We find a significant correlation between median core flux density and $\gamma$-ray flux, but no correlation with typical Doppler boosting indicators such as median core brightness temperature and core dominance. Our study suggests that high-energy emission from radio galaxies is related to parsec-scale radio emission from the inner jet, but is not driven by Doppler boosting effects, in contrast to the situation in their blazar counterparts., Comment: Main text: 15 pages, 12 figures, 6 tables. Appendix: 14 pages, 20 figures, 13 tables. Accepted for publication in Astronomy and Astrophysics

As we move into the next era of geodetic VLBI, the scheduling process is one focus for improvement in terms of increased flexibility and the ability to react with changing conditions. A range of simulations were conducted to ascertain the impact of scheduling on geodetic results such as Earth Orientation Parameters (EOPs) and station coordinates. The potential capabilities of new automated scheduling modes were also simulated, using the so-called ‘dynamic scheduling’ technique. The primary aim was to improve efficiency for both cost and time without losing geodetic precision, particularly to maximise the uses of the Australian AuScope VLBI array. We show that short breaks in observation will not significantly degrade the results of a typical 24 h experiment, whereas simply shortening observing time degrades precision exponentially. We also confirm the new automated, dynamic scheduling mode is capable of producing the same standard of result as a traditional schedule, with close to real-time flexibility. Further, it is possible to use the dynamic scheduler to augment the 3 station Australian AuScope array and thereby attain EOPs of the current global precision with only intermittent contribution from 2 additional stations. We thus confirm automated, dynamic scheduling bears great potential for flexibility and automation in line with aims for future continuous VLBI operations.

The APOD (Atmospheric density detection and Precise Orbit Determination) is the first LEO (Low Earth Orbit) satellite in orbit co-located with a dual-frequency GNSS (GPS/BD) receiver, an SLR reflector, and a VLBI X/S dual band beacon. From the overlap statistics between consecutive solution arcs and the independent validation by SLR measurements, the orbit position deviation was below 10 cm before the on-board GNSS receiver got partially operational. In this paper, the focus is on the VLBI observations to the LEO satellite from multiple geodetic VLBI radio telescopes, since this is the first implementation of a dedicated VLBI transmitter in low Earth orbit. The practical problems of tracking a fast moving spacecraft with current VLBI ground infrastructure were solved and strong interferometric fringes were obtained by cross-correlation of APOD carrier and DOR (Differential One-way Ranging) signals. The precision in X-band time delay derived from 0.1 s integration time of the correlator output is on the level of 0.1 ns. The APOD observations demonstrate encouraging prospects of co-location of multiple space geodetic techniques in space, as a first prototype.

The possibility of observing satellites with the very long baseline interferometry (VLBI) technique has been discussed for several years in the geodetic community, with observations of either existing satellites of the global navigation satellite systems or of satellites dedicated to realise a space tie. Such observations were carried out using the Australian telescopes in Hobart and Ceduna which, for the first time, integrated all the necessary steps: planning the observations (automated scheduling), correlation of the data and the generation of a series of time delay observables suitable for a subsequent geodetic analysis. We report on the development of new and the adaptation of existing routines for observing and data processing, focusing on technology development. The aim was to use methods that are routinely used in geodetic VLBI. A series of test experiments of up to six hours duration was performed, allowing to improve the observations from session to session and revealing new problems still to be solved. The newly developed procedures and programs now enable more observations. Further development assumed, this bears the prospect of being directly applied to the observation of dedicated space-tie satellites.

Context. We highlight the capabilities of geodetic VLBI technique to test general relativity in the classical astrometric style, i.e. measuring the deflection of light in the vicinity of the Sun.Aims. In previous studies, the parameterγwas estimated by global analyses of thousands of geodetic VLBI sessions. Here we estimateγfrom a single session where the Sun has approached two strong reference radio sources, 0229+131 and 0235+164, at an elongation angle of 1–3°.Methods. The AUA020 VLBI session of 1 May 2017 was designed to obtain more than 1000 group delays from the two radio sources. The solar corona effect was effectively calibrated with the dual-frequency observations even at small elongation.Results. We obtainedγwith a greater precision (0.9 × 10−4) than has been obtained through global analyses of thousands of standard geodetic sessions over decades. Current results demonstrate that the modern VLBI technology is capable of establishing new limits on observational tests of general relativity.

We present an approach to testing the gravitational redshift effect using the RadioAstron satellite. The experiment is based on a modification of the Gravity Probe A scheme of nonrelativistic Doppler compensation and benefits from the highly eccentric orbit and ultra-stable atomic hydrogen maser frequency standard of the RadioAstron satellite. Using the presented techniques we expect to reach an accuracy of the gravitational redshift test of order $10^{-5}$, a magnitude better than that of Gravity Probe A. Data processing is ongoing, our preliminary results agree with the validity of the Einstein Equivalence Principle., Comment: Submitted to Physics Letters A

We report the results of Long Baseline Array observations made in 2001 of ten southern sources proposed by Mattox et al. as counterparts to EGRET >100 MeV gamma-ray sources. Source structures are compared with published data where available and possible superluminal motions identified in several cases. The associations are examined in the light of Fermi observations, indicating that the confirmed counterparts tend to have radio properties consistent with other identifications, including flat radio spectral index, high brightness temperature, greater radio variability, and higher core dominance.

Thespatialstructureofsourcesmakingupthecelestialreferenceframe(CRF)atradiofrequencies is a systematic error source in very long baseline interferometry (VLBI) measurements. Using simulations, we investigate the effects of source structure on the CRF, determined by the actual observational programme of the International VLBI Service for Geodesy and Astrometry. This is done using the source structure simulator of the Vienna VLBI Software. Applying various mock two-component source models, systematic displacements of 10–80 μas in median source position offsets are found. These offsets are predominantly aligned with the direction of the jet. The simulations further show that slight changes in the source model can significantly change the estimated positions. We finally present a new parametrization of sourcepositionsintheanalysis,namelyalongthejetdirectionandperpendiculartoit,allowing us to significantly mitigate the effects of source structure on an estimated CRF.

Inhomogeneous networks and reference frames are an important issue for Very Long Baseline Interferometry (VLBI). In this work we examine the performance of southern stations and baselines in routine VLBI experiments. A positive impact on baseline length repeatabilities of the increased observing effort by the Australian AuScope VLBI antennas is found by analysing three years of global rapid-turnaround VLBI sessions: while worse results are found for southern baselines compared to northern baselines for the first half of the investigated sessions, the northern and southern baseline length repeatabilities are about the same in the second half of the period. In simulations, the actual observing plan with a significantly lower number of observations for southern stations is identified as a major reason for the worse length WRMS for southern baselines, though other factors seem to influence the results as well. Simulating radio source position uncertainties, effects of up to 10 mm are found on baseline length WRMS for long southern baselines. Improving all source position uncertainties through more frequent observations to better than 50 μ as could reduce this effect by up to 30 % .

We investigate the relationship between 5 GHz interstellar scintillation (ISS) and 15 GHz intrinsic variability of compact, radio-selected AGNs drawn from the Microarcsecond Scintillation-Induced Variability (MASIV) Survey and the Owens Valley Radio Observatory (OVRO) blazar monitoring program. We discover that the strongest scintillators at 5 GHz (modulation index, $m_5 \geq 0.02$) all exhibit strong 15 GHz intrinsic variability ($m_{15} \geq 0.1$). This relationship can be attributed mainly to the mutual dependence of intrinsic variability and ISS amplitudes on radio core compactness at $\sim 100\, \mu$as scales, and to a lesser extent, on their mutual dependences on source flux density, arcsec-scale core dominance and redshift. However, not all sources displaying strong intrinsic variations show high amplitude scintillation, since ISS is also strongly dependent on Galactic line-of-sight scattering properties. This observed relationship between intrinsic variability and ISS highlights the importance of optimizing the observing frequency, cadence, timespan and sky coverage of future radio variability surveys, such that these two effects can be better distinguished to study the underlying physics. For the full MASIV sample, we find that Fermi-detected gamma-ray loud sources exhibit significantly higher 5 GHz ISS amplitudes than gamma-ray quiet sources. This relationship is weaker than the known correlation between gamma-ray loudness and the 15 GHz variability amplitudes, most likely due to jet opacity effects., Comment: 18 pages, 13 figures, 5 tables, resubmitted to MNRAS after minor revision

We present here the results of the first part of the VLBI Ecliptic Plane Survey (VEPS) program. The goal of the program is to find all compact sources within $7.5^\circ$ of the ecliptic plane which are suitable as calibrators for anticipated phase referencing observations of spacecraft and determine their positions with accuracy at the 1.5~nrad level. We run the program in two modes: the search mode and the refining mode. In the search mode, a complete sample of all sources brighter than 50 mJy at 5 GHz listed in the Parkes-MIT-NRAO (PMN) and Green Bank 6~cm (GB6) catalogs, except those previously detected with VLBI, is observed. In the refining mode, the positions of all ecliptic plane sources, including those found in the search mode, are improved. By October 2016, thirteen 24-hr sessions that targeted all sources brighter than 100~mJy have been observed and analyzed. Among 3320 observed target sources, 555 objects have been detected. We also conducted a number of follow-up VLBI experiments in the refining mode and improved the positions of 249 ecliptic plane sources., 12 pages, 5 figures, 5 tables, accepted by The Astrophysical Journal Supplement Series (ApJS)

We present results from a multiwavelength study of the blazar PKS 1954-388 at radio, UV, X-ray, and gamma-ray energies. A RadioAstron observation at 1.66 GHz in June 2012 resulted in the detection of interferometric fringes on baselines of 6.2 Earth-diameters. This suggests a source frame brightness temperature of greater than 2x10^12 K, well in excess of both equipartition and inverse Compton limits and implying the existence of Doppler boosting in the core. An 8.4 GHz TANAMI VLBI image, made less than a month after the RadioAstron observations, is consistent with a previously reported superluminal motion for a jet component. Flux density monitoring with the Australia Telescope Compact Array confirms previous evidence for long-term variability that increases with observing frequency. A search for more rapid variability revealed no evidence for significant day-scale flux density variation. The ATCA light-curve reveals a strong radio flare beginning in late 2013 which peaks higher, and earlier, at higher frequencies. Comparison with the Fermi gamma-ray light-curve indicates this followed ~9 months after the start of a prolonged gamma-ray high-state -- a radio lag comparable to that seen in other blazars. The multiwavelength data are combined to derive a Spectral Energy Distribution, which is fitted by a one-zone synchrotron-self-Compton (SSC) model with the addition of external Compton (EC) emission., Comment: 12 pages, 7 figures

The International Celestial Reference Frame suffers from significantly less observations in the southern hemisphere compared to the northern one. One reason for this is the historically low number of very long baseline interferometry radio telescopes in the south. The AuScope very long baseline interferometry array with three new telescopes on the Australian continent and an identical antenna in New Zealand were built to address this issue. While the overall number of observations in the south has greatly improved since then, a closer look reveals that this improvement is only true for strong radio sources (source flux densities >0.6 Jy). The new array of small very long baseline interferometry antennas has a relatively low baseline sensitivity so that only strong sources can be observed within a short integration time. A new observing strategy, the star scheduling mode, was developed to enable efficient observations of weak sources during geodetic sessions, through the addition of a single more sensitive antenna to the network. This scheduling mode was implemented in the Vienna very long baseline interferometry Software and applied in four 24-h sessions in 2016. These observations provide updated positions and source flux densities for 42 weak southern radio sources and significantly reduce the formal uncertainties for these sources. The star scheduling mode now allows the AuScope very long baseline interferometry array to undertake greater responsibility in monitoring sources in the southern sky, without significantly weakening the session for geodetic purposes.

We have completed a Chandra snapshot survey of 54 radio jets that are extended on arcsec scales. These are associated with flat spectrum radio quasars spanning a redshift range z=0.3 to 2.1. X-ray emission is detected from the jet of approximately 60% of the sample objects. We assume minimum energy and apply conditions consistent with the original Felten-Morrison calculations in order to estimate the Lorentz factors and the apparent Doppler factors. This allows estimates of the enthalpy fluxes, which turn out to be comparable to the radiative luminosities., Comment: Conference Proceedings IAU Symposium No. 313, Extragalactic jets from every angle, pp. 219-224, 4 figures

Observations of the frequencies of different rotational transitions of the methanol molecule have provided the most sensitive probe to date for changes in the proton-to-electron mass ratio, over space and time. Using methanol absorption detected in the gravitational lens system PKS B1830-211, changes in the proton-to-electron ratio over the last 7.5 billion years have been constrained to a fractional change less than 1.1e-07. Molecular absorption systems at cosmological distances present the best opportunity for constraining or measuring changes in the fundamental constants of physics over time, however, we are now at the stage where potential differences in the morphology of the absorbing systems and the background source, combined with their temporal evolution, provide the major source of uncertainty in some systems. Here we present the first milliarcsecond resolution observations of the molecular absorption system towards PKS B1830-211. We have imaged the absorption from the 12.2-GHz transition of methanol (which is redshifted to 6.45 GHz) toward the southwestern component and show that it is possibly offset from the peak of the continuum emission and partially resolved on milliarcsecond scales. Future observations of other methanol transitions with similar angular resolution offer the best prospects for reducing systematic errors in investigations of possible changes in the proton-to-electron mass ratio on cosmological scales., 8 pages, 3 figures, accepted for publication in MNRAS

The discovery of the first quasar 3C 273 led directly to the discovery of their variability at optical and radio wavelengths. We review the radio variability observations, in particular the variability found at frequencies below 1 GHz, as well as those exhibiting intra-day variability (IDV) at cm wavelengths. Observations have shown that IDV arises principally from scintillation caused by scattering in the ionized interstellar medium of our Galaxy. The sensitivity of interstellar scintillation towards source angular sizes has provided a powerful tool for studying the most compact components of radio-loud AGN at microarcsecond and milliarcsecond scale resolution.

We examine the relationship between astrometric stability and astrophysical properties in flat-spectrum radio-loud quasars making up the celestial reference frame. We use position determinations from geodetic very long baseline interferometry measurements and develop a new metric for source position stability. We then compare this quantity to two measures of source compactness: structure index, which probes structure on milliarcsecond scales and the presence of interstellar scintillation associated with the quasar, which probes scales of tens of μas. We find that persistent scintillators have greater position stability than episodic scintillators, which are in turn more stable than non-scintillators. Scintillators are also more likely to be compact on milliarcsecond scales, as measured by the structure index. Persistent scintillators are therefore excellent candidates for inclusion in any future realization of the celestial reference frame. A list of these sources is presented in Appendix A. We find that slow (characteristic time-scale >3 d) scintillators have more stable positions than rapid scintillators, suggesting that they are more compact. High-cadence, long-term monitoring is therefore required to identify other members of this population of compact, high brightness temperature quasars.

We report the detection of four water masers within the Small Magellanic Cloud (SMC): two discovered with the 70-m Tidbinbilla radio telescope, and two discovered with the Australia Telescope Compact Array (ATCA). Precise positions of all four masers have been derived from ATCA observations, and the characteristics of each water maser have been monitored over a period of several years. Sensitive observations towards two previously detected water masers reported in the literature failed to detect any emission. The detected water masers show evidence of higher levels of temporal variability than equivalent Galactic sources, and one of the features associated with NGC 346 IR1 shows an acceleration of 9.6 km s −1 yr −1 over a 31 d period. Sensitive targeted observations for methanol and OH masers failed to detect any accompanying emission – in the case of methanol perhaps highlighting an under abundance in the SMC, consistent with expectations due to lower metallicity. The water masers are both bright and compact making them excellent targets for Very Long Baseline Interferometry (VLBI) observations, which can potentially measure the proper motion of the SMC (∼1–2 mas yr −1 ) with temporal baselines of ∼12 months. Such observations would utilize sources associated with only the current epoch of star formation and hence have several advantages over alternative methods.

The closest ever fly-by of the Martian moon Phobos, performed by the European Space Agency's Mars Express spacecraft, gives a unique opportunity to sharpen and test the Planetary Radio Interferometry and Doppler Experiments (PRIDE) technique in the interest of studying planet - satellite systems. The aim of this work is to demonstrate a technique of providing high precision positional and Doppler measurements of planetary spacecraft using the Mars Express spacecraft. The technique will be used in the framework of Planetary Radio Interferometry and Doppler Experiments in various planetary missions, in particular in fly-by mode. We advanced a novel approach to spacecraft data processing using the techniques of Doppler and phase-referenced very long baseline interferometry spacecraft tracking. We achieved, on average, mHz precision (30 {\mu}m/s at a 10 seconds integration time) for radial three-way Doppler estimates and sub-nanoradian precision for lateral position measurements, which in a linear measure (at a distance of 1.4 AU) corresponds to ~50 m., Comment: 9 pages, 14 figures. Astronomy and Astrophysics, accepted on 2016/05/31

The discovery of extraterrestrial very-high-energy neutrinos by the IceCube collaboration has launched a quest for the identification of their astrophysical sources. Gamma-ray blazars have been predicted to yield a cumulative neutrino signal exceeding the atmospheric background above energies of 100 TeV, assuming that both the neutrinos and the gamma-ray photons are produced by accelerated protons in relativistic jets. Since the background spectrum falls steeply with increasing energy, the individual events with the clearest signature of being of an extraterrestrial origin are those at PeV energies. Inside the large positional-uncertainty fields of the first two PeV neutrinos detected by IceCube, the integrated emission of the blazar population has a sufficiently high electromagnetic flux to explain the detected IceCube events, but fluences of individual objects are too low to make an unambiguous source association. Here, we report that a major outburst of the blazar PKS B1424-418 occurred in temporal and positional coincidence with the third PeV-energy neutrino event (IC35) detected by IceCube. Based on an analysis of the full sample of gamma-ray blazars in the IC35 field and assuming a photo-hadronic emission model, we show that the long-term average gamma-ray emission of blazars as a class is in agreement with both the measured all-sky flux of PeV neutrinos and the spectral slope of the IceCube signal. The outburst of PKS B1424-418 has provided an energy output high enough to explain the observed PeV event, indicative of a direct physical association., Accepted for publication in Nature Physics on Mar 3rd, 2016 (arXiv version is the original submitted manuscript as per NatPhys regulations), 38 pages (including 10 pages of supplementary material), 3 figures

We thank the referee for helpful comments. We thank S. Cutini for her useful comments. We thank S. Markoff for helpful discussions. We thank J. Perkins, L. Baldini, and S. Digel for carefully reading the manuscript. We thank M. Buxton for her help with the SMARTS data. We acknowledge support and partial funding by the Deutsche Forschungsgemeinschaft grant WI 1860-10/1 (TANAMI) and GRK 1147, Deutsches Zentrum fur Luft- und Raumfahrt grants 50 OR 1311 and 50 OR 1103, and the Helmholtz Alliance for Astroparticle Physics (HAP). This research was funded in part by NASA through Fermi Guest Investigator grants NNH09ZDA001N, NH10ZDA001N, NNH12ZDA001N, and NNH13ZDA001N-FERMI. This research was supported by an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by Oak Ridge Associated Universities through a contract with NASA. E.R. was partially supported by the Spanish MINECO project AYA2012-38491-C02-01 and by the Generalitat Valenciana project PROMETEO II/2014/057. We thank J. E. Davis for the development of the slxfig module that was used to prepare the figures in this work. We thank T. Johnson for the Fermi/LAT SED scripts, which were used to calculate the Fermi/LAT spectra. This research has made use of a collection of ISIS scripts provided by the Dr. Karl Remeis-Observatory, Bamberg, Germany at http://www.sternwarte.uni-erlangen.de/isis/. The Long Baseline Array and Australia Telescope Compact Array are part of the Australia Telescope National Facility, which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO. This paper has made use of up-to-date SMARTS optical/near-infrared light curves that are available at www.astro.yale.edu/smarts/glast/home.php. The Fermi LAT Collaboration acknowledges generous ongoing support from a number of agencies and institutes that have supported both the development and the operation of the LAT as well as scientific data analysis. These include the National Aeronautics and Space Administration and the Department of Energy in the United States, the Commissariat a l’Energie Atomique and the Centre National de la Recherche Scientifique / Institut National de Physique Nucleaire et de Physique des Particules in France, the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), High Energy Accelerator Research Organization (KEK) and Japan Aerospace Exploration Agency (JAXA) in Japan, and the K.A. Wallenberg Foundation, the Swedish Research Council and the Swedish National Space Board in Sweden. Additional support for science analysis during the operations phase is gratefully acknowledged from the Istituto Nazionale di Astrofisica in Italy and the Centre National d’Etudes Spatiales in France

The flat spectrum radio quasar (FSRQ) PKS 2123-463 was associated in the First Fermi-LAT source catalog with the gamma-ray source 1FGL J2126.1-4603, but when considering the full first two years of Fermi observations, no gamma-ray source at a position consistent with this FSRQ was detected, and thus PKS 2123-463 was not reported in the Second Fermi-LAT source catalog. On 2011 December 14 a gamma-ray source positionally consistent with PKS 2123-463 was detected in flaring activity by Fermi-LAT. This activity triggered radio-to-X-ray observations by the Swift, GROND, ATCA, Ceduna, and KAT-7 observatories. Results of the localization of the gamma-ray source over 41 months of Fermi-LAT operation are reported here in conjunction with the results of the analysis of radio, optical, UV and X-ray data collected soon after the gamma-ray flare. The strict spatial association with the lower energy counterpart together with a simultaneous increase of the activity in optical, UV, X-ray and gamma-ray bands led to a firm identification of the gamma-ray source with PKS 2123-463. A new photometric redshift has been estimated as z = 1.46+/-0.05 using GROND and Swift/UVOT observations, in rough agreement with the disputed spectroscopic redshift of z = 1.67. We fit the broadband spectral energy distribution with a synchrotron/external Compton model. We find that a thermal disk component is necessary to explain the optical/UV emission detected by Swift/UVOT. This disk has a luminosity of about 1.8x10^46 erg/s, and a fit to the disk emission assuming a Schwarzschild (i.e., nonrotating) black hole gives a mass of about 2x10^9 solar masses. This is the first black hole mass estimate for this source.

The Australia Telescope Large Area Survey (ATLAS) has surveyed seven square degrees of sky around the Chandra Deep Field South (CDFS) and the European Large Area ISO Survey - South 1 (ELAIS-S1) fields at 1.4 GHz. ATLAS aims to reach a uniform sensitivity of $10 \mu$Jy beam$^{-1}$ rms over the entire region with data release 1 currently reaching $\sim30 \mu$Jy beam$^{-1}$ rms. Here we present 466 new spectroscopic redshifts for radio sources in ATLAS as part of our optical follow-up program. Of the 466 radio sources with new spectroscopic redshifts, 142 have star-forming optical spectra, 282 show evidence for AGN in their optical spectra, 10 have stellar spectra and 32 have spectra revealing redshifts, but with insufficient features to classify. We compare our spectroscopic classifications with two mid-infrared diagnostics and find them to be in broad agreement. We also construct the radio luminosity function for star-forming galaxies to z $= 0.5$ and for AGN to z $= 0.8$. The radio luminosity function for star-forming galaxies appears to be in good agreement with previous studies. The radio luminosity function for AGN appears higher than previous studies of the local AGN radio luminosity function. We explore the possibility of evolution, cosmic variance and classification techniques affecting the AGN radio luminosity function. ATLAS is a pathfinder for the forthcoming EMU survey and the data presented in this paper will be used to guide EMU's survey design and early science papers.

Using the Australia Telescope Compact Array (ATCA), ATLAS (Australia Telescope Large Area Survey) is imaging two fields totalling 7 square degrees down to 10 μJy beam − 1 at 1.4 GHz. We have found 6 wide-angle tail galaxies (WATs), 4 of which have sufficient data to identify associated galaxy overdensities. The largest WAT, at a red-shift of 0.22, appears to be associated with an overdensity of galaxies that is spread over an unusually large extent of 12 Mpc, with a velocity range of 4500 km s − 1. Here we present the WATs in ATLAS and discuss the implications of these observations for future large-scale radio surveys such as ASKAP-EMU.

We present radio images of a sample of six Wide-Angle Tail (WAT) radio sources identified in the ATLAS 1.4 GHz radio survey, and new spectroscopic redshifts for four of these sources. These WATs are in the redshift range of 0.1469 - 0.3762, and we find evidence of galaxy overdensities in the vicinity of four of the WATs from either spectroscopic or photometric redshifts. We also present follow-up spectroscopic observations of the area surrounding the largest WAT, S1189, which is at a redshift of ~0.22. The spectroscopic observations, taken using the AAOmega spectrograph on the AAT, show an overdensity of galaxies at this redshift. The galaxies are spread over an unusually large area of ~12 Mpc with a velocity spread of ~4500 km/s. This large-scale structure includes a highly asymmetric FRI radio galaxy and also appears to host a radio relic. It may represent an unrelaxed system with different sub-structures interacting or merging with one another. We discuss the implications of these observations for future large-scale radio surveys.

We summarize a study of PKS 0920-397 using our 42 ks Chandra observation in conjunction with our ATCA 20GHz image, and HST/ACS F814W and F475W images. We investigate the hypothesis that the jet X–ray emission is due to inverse-Compton (IC) scattering on the cosmic microwave background (CMB) from the same population of relativistic electrons that give rise to the radio emission. To calculate parameters intrinsic to the source, one must finesse the fact that we do not know the true angle of the jet to our line of sight. Typical assumptions are that the Doppler factor equals the bulk Lorentz factor, or that the Lorentz factor takes some fixed numerical value. While giving useful estimates, neither assumption can be exact in general. We try different constraints to determine the jet quantities. It is plausible that the kinetic flux is constant along the jet, prior to a terminal hotspot or lobe, and with minimal bending of the jet. Alternatively, because PKS 0920-397 appears straight in projection on the sky, we might assume the jet maintains a constant angle to our line of sight. Either approach gives bulk Lorentz factors of 6 to 8, with kinetic energy flux of order 1046 erg s-1, and with the jet at an angle 2° to 4° from our line of sight.

We have used the Long Baseline Array to observe the Circinus Galaxy H2O megamasers on two epochs separated by 49 d. The masers were observed to be highly variable, consistent with previous studies. We observe rapid variations in the linewidth and central velocity of the maser's spectral profile. The characteristics of these spectral variations suggest that the masers have an anisotropic structure, with axial ratios ≲5, and display velocity gradients in their microarcsecond structure. We also find a strong anticorrelation between the linewidth and the modulation index of the maser features and a relationship between the modulation index and characteristic time-scale of the variations. These various phenomena can all be simply explained through a model where the rapid variations in the masers are produced by quenched diffractive scintillation in the presence of modest velocity gradients in the maser emission. The very long baseline interferometry (VLBI) images are consistent with previous high-resolution observations and show maser emission arising both in a circumnuclear accretion disc, and in a low-velocity outflow. We find no systematic differences in the variability properties of the disc and outflow masers. The VLBI images reveal that there is considerable spectral blending in single-dish spectra of the Circinus megamasers. In addition to the VLBI imaging we also used these observations to look for a time delay in the intensity variations observed at the different telescopes in the array, as expected for scintillation-induced variability. No time delay was detected, which allows us to place a lower limit on the velocity of the interstellar medium during these observations 22 km s−1 for the second epoch. We conclude that any future attempts to measure a time delay in this source should utilize intercontinental baselines to increase the chances of success.

In November 2006, the Australian Federal Government announced $15.8M in funding for geospatial research infrastructure through the National Collaborative Research Infrastructure Strategy (NCRIS). NCRIS is an initiative under the Australian Government?s Backing Australia?s Ability package with a number of key principles, including maximising the contributions of the R&D system to economic development, national security, social wellbeing and environmental sustainability. Here we outline why particular components of geospatial infrastructure are required in Australia to advance (equip) geospatial research over the next 20 years. We describe some of the scientific objectives that required an upgrade and densification of Australia?s geospatial infrastructure. This paper is the perspective of researchers from a subset of University researchers involved in the AuScope Geospatial component, and so does not necessarily encompass the opinions of all those involved in AuScope geospatial.

In February 1997, the Japanese radio astronomy satellite HALCA was launched to provide the space-bourne element for the VLBI Space Observatory Programme (VSOP) mission. Approximately twenty-five percent of the mission time was dedicated to the VSOP Survey of bright compact Active Galactic Nuclei (AGN) at 5 GHz. This paper, the fifth in the series, presents images and models for the remaining 140 sources not included in Paper III, which contained 102 sources. For most sources, the plots of the uv-coverage, the visibility amplitude versus uv-distance, and the high resolution image are presented. Model fit parameters to the major radio components are determined, and the brightness temperature of the core component for each source is calculated. The brightness temperature distributions for all of the sources in the VSOP AGN survey are discussed., Comment: Accepted ApJS

We present observations of a rare, rapid, high amplitude Extreme Scattering Event toward the compact BL-Lac AO 0235+164 at 6.65 GHz. The ESE cloud is compact; we estimate its diameter between 0.09 and 0.9 AU, and is at a distance of less than 3.6 kpc. Limits on the angular extent of the ESE cloud imply a minimum cloud electron density of ~ 4 x 10^3 cm^-3. Based on the amplitude and timescale of the ESE observed here, we suggest that at least one of the transients reported by Bower et al. (2007) may be attributed to ESEs., Comment: 11 pages, 2 figures

We present the results of a large-scale 5-GHz Very Large Array (VLA) survey of the northern sky searching for sources that exhibit intraday variability (IDV). The observations were made over four epochs, each of 3 days in January, May and September 2002 and January 2003. The objective is to obtain a large sample of IDV sources in order to derive reliable statistics of the microarcsecond properties of the source population and the scattering properties of the interstellar medium.

Gamma-ray detected radio-loud narrow-line Seyfert 1 (g-NLS1) galaxies constitute a small but interesting sample of the gamma-ray loud AGN. The radio-loudest g-NLS1 known, PKS 2004-447, is located in the southern hemisphere and is monitored in the radio regime by the multiwavelength monitoring program TANAMI. We aim for the first detailed study of the radio morphology and long-term radio spectral evolution of PKS 2004-447, which are essential to understand the diversity of the radio properties of g-NLS1s. The TANAMI VLBI monitoring program uses the Australian Long Baseline Array (LBA) and telescopes in Antarctica, Chile, New Zealand, and South Africa to monitor the jets of radio-loud active galaxies in the southern hemisphere. Lower resolution radio flux density measurements at multiple radio frequencies over four years of observations were obtained with the Australia Telescope Compact Array (ATCA). The TANAMI VLBI image at 8.4 GHz shows an extended one-sided jet with a dominant compact VLBI core. Its brightness temperature is consistent with equipartition, but it is an order of magnitude below other g-NLS1s with the sample value varying over two orders of magnitude. We find a compact morphology with a projected large-scale size, 11 pages, 4 figures, accepted by Astronomy & Astrophysics

The Chandra X-ray observatory has discovered dozens of resolved, kiloparsec-scale jets associated with powerful quasars in which the X-ray fluxes are observed to be much higher than the expected level based on the radio-optical synchrotron spectrum. The most popular explanation for the anomalously high and hard X-ray fluxes is that these jets do not decelerate significantly by the kiloparsec scale, but rather remain highly relativistic (Lorentz factors $\Gamma\approx$10). By adopting a small angle to the line-of-sight, the X-rays can thus be explained by inverse Compton upscattering of CMB photons (IC/CMB), where the observed emission is strongly Doppler boosted. Using over six years of Fermi monitoring data, we show that the expected hard, steady gamma-ray emission implied by the IC/CMB model is not seen in PKS 0637-752, the prototype jet for which this model was first proposed. IC/CMB emission is thus ruled out as the source of the X-rays, joining recent results for the jets in 3C 273 (using the same method; Meyer et al. 2014) and PKS 1136-135 (using UV polarization; Cara et al., 2013). We further show that the Fermi observations give an upper limit of $\delta, Comment: 8 pages, 7 figures, accepted for publication in ApJ

The AUSTRAL observing program is an initiative led by the Australian AuScope VLBI antennas in collaboration with radio telescopes in Warkworth, New Zealand, and Hartebeesthoek, South Africa. In 2014 the number of AUSTRAL sessions increased tremendously. Comparing recent results to the standard products achieved in global VLBI sessions regularly undertaken by the International VLBI Service for Geodesy and Astrometry (IVS), better accuracies in terms of baseline length repeatabilities are found for these regional AUSTRAL sessions. The network of (almost) identical small and fast telescopes as well as the technical equipment at all stations allows for new observing modes and improved operations, as such serving as a testbed for the future VLBI Global Observing System (VGOS). Special AUST-Astro sessions are used for dedicated astrometry of sparsely observed radio sources in the southern sky, as well as for detecting new radio sources for geodesy. In 2015, the AUSTRAL program will be further increased and final steps are now being undertaken for full VGOS compatibility of the three AuScope VLBI antennas. We present the latest results of the AUSTRAL sessions and give an overview of the multiple areas of research they support.

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.

Multiwavelength Very Long Baseline Array observations of a sample of 49 extragalactic radio sources selected from the Micro-Arcsecond Scintillation-Induced Variability survey are presented. These observations are intended to provide a sizeable, uniform data set for the investigation of angular broadening of scintillating sources. These data will be used to compare and contrast the scattering behavior of scintillating and nonscintillating radio sources in subsequent analysis. We confirm earlier analyses that found scintillating sources to be more core-dominated than nonscintillating sources.

Accepted: 2005-11-13, 資料番号: SA1000482000

We discuss the physical properties of four quasar jets imaged with the Chandra X-ray Observatory in the course of a survey for X-ray emission from radio jets. These objects have sufficient counts to study their spatially resolved properties, even in the 5 ks survey observations. We have acquired Australia Telescope Compact Array data with resolution matching Chandra. We have searched for optical emission with Magellan, with sub-arcsecond resolution. The radio to X-ray spectral energy distribution for most of the individual regions indicates against synchrotron radiation from a single-component electron spectrum. We therefore explore the consequences of assuming that the X-ray emission is the result of inverse Compton scattering on the cosmic microwave background. If particles and magnetic fields are near minimum energy density in the jet rest frames, then the emitting regions must be relativistically beamed, even at distances of order 500 kpc from the quasar. We estimate the magnetic field strengths, relativistic Doppler factors, and kinetic energy flux as a function of distance from the quasar core for two or three distinct regions along each jet. We develop, for the first time, estimates in the uncertainties in these parameters, recognizing that they are dominated by our assumptions in applying the standard synchrotron minimum energy conditions. The kinetic power is comparable with, or exceeds, the quasar radiative luminosity, implying that the jets are a significant factor in the energetics of the accretion process powering the central black hole. The measured radiative efficiencies of the jets are of order 10^(-4)., Comment: Accepted for Publication in the Astrophysical Journal, Part I

We report the discovery of water maser emission in eight active galactic nuclei (AGN) with the 70-m NASA Deep Space Network (DSN) antennas at Tidbinbilla, Australia and Robledo, Spain. The positions of the newly discovered masers, measured with the VLA, are consistent with the optical positions of the host nuclei to within 1 sigma (0.3 arcsec radio and 1.3 arcsec optical) and most likely mark the locations of the embedded central engines. The spectra of two sources, NGC 3393 and NGC 5495, display the characteristic spectral signature of emission from an edge-on accretion disk, with orbital velocities of ~600 and ~400 km s^-1, respectively. In a survey with DSN facilities of 630 AGN selected from the NASA Extragalactic Database, we have discovered a total of 15 water maser sources. The resulting incidence rate of maser emission among nearby (v_sys < 7000 km s^-1) Seyfert 1.8-2.0 and LINER systems is ~10 percent for a typical rms noise level of ~14 mJy over 1.3 km s^-1 spectral channels. As a result of this work, the number of nearby AGN (v_sys < 7000 km s^-1) observed with, 7 pages, 1 figure, 2 tables, to appear in the 2006 February 10 issue of the Astrophysical Journal, uses twocolumns.cls

We present radio continuum observations of the planetary nebula (PN) IRAS 17347-3139 (one of the only two known to harbour water maser emission), made to derive its spectral index and the turnover frequency of the emission. The spectrum of the source rises in the whole frequency range sampled, from 2.4 to 24.9 GHz, although the spectral index seems to decrease at the highest frequencies (0.79+-0.04 between 4.3 and 8.9 GHz, and 0.64+-0.06 between 16.1 and 24.9 GHz). This suggests a turnover frequency around 20 GHz (which is unusual among PNe, whose radio emission usually becomes optically thin at frequencies < 10 GHz), and a relatively high emission measure (1.5 x 10^9 cm^{-6} pc). The radio continuum emission has increased by a factor of ~1.26 at 8.4 GHz in 13 years, which can be explained as expansion of the ionized region by a factor of ~1.12 in radius with a dynamical age of ~120 yr and at an expansion velocity of ~5-40 km/s. These radio continuum characteristics, together with the presence of water maser emission and a strong optical extinction suggest that IRAS 17347-3139 is one of the youngest PNe known, with a relatively massive progenitor star., Five pages, 2 figures, accepted by MNRAS

Interstellar scintillation has been conclusively demonstrated to be the principal cause of the intraday variability (IDV) observed in the centimeter-wavelength emission of many active galactic nuclei. A few sources show large amplitude modulation in their flux density on a timescale of hours. However, the majority of IDV sources exhibit variability on timescales of a day or more. Some sources have been found to show an annual cycle in the pattern of their variability. Such an annual cycle occurs because the relative speeds of the Earth and the interstellar medium change as the Earth orbits the Sun. To search for these annual variations, as well as to follow the source evolution, requires a dedicated instrument; the necessary amounts of observing time are beyond the capability of the National Facility instruments. Here we describe the scientific motivation for and present an outline of the COSMIC (Continuous Single-Dish Monitoring of Intraday Variability at Ceduna) project, which uses the University of Tasmania's 30 m diameter radio telescope at Ceduna, which has been monitoring the flux density of a number of the stronger southern scintillators at 6.65 GHz since 2003 March.

PKS 1257—326 is one of three quasars known to show unusually large and rapid, intra-hour intensity variations, as a result of scintillation in the turbulent Galactic interstellar medium. We have measured time delays in the variability pattern arrival times at the VLA and the ATCA, as well as an annual cycle in the time-scale of variability for this source. Results of the two-station time delay observations are presented here. Implications for the scintillation of this source are discussed in the light of these results, together with results from two years of monitoring with the ATCA.

We compare the milliarcsecond-scale morphology of scintillating and nonscintillating sources. The scintillating sources are drawn from those flat-spectrum extragalactic radio sources discovered, by the Micro-Arcsecond Scintillation-Induced Variability Survey, to have flux density variability at 5 GHz on timescales from hours to days. Intrinsic source structure information is obtained from previously published and/or publicly available 8.4 GHz Very Long Baseline Array images. A sample of low flux density (Sν=5 GHz < 0.3 Jy) scintillating sources was compared with a sample of high flux density (Sν=5 GHz ~ 1 Jy) scintillators, as well as a sample of high flux density nonscintillators. All source samples meet the selection criteria of the Micro-Arcsecond Scintillation-Induced Variability Survey, thus ensuring that all three source samples are suitable for comparative study. We find that all scintillating sources (both low and high flux density samples) are significantly more core dominated than nonscintillating sources. Further, the overall source size of the scintillating sources is significantly smaller than that of nonscintillators. There does not appear to be any significant difference between the milliarcsecond-scale morphologies of low and high flux density scintillators. These results demonstrate that it is the core of the radio source that is scintillating.

We present 8.4 GHz Very Long Baseline Array (VLBA) observations of 75 extragalactic radio sources drawn from the scintillating sources discovered in the Microarcsecond Scintillation-induced Variability survey. This survey clearly showed that the fraction of scintillating sources and the amplitude of their variability increases strongly with decreasing flux density, raising the possibility that scintillating sources may be systematically extremely core dominated. The observations presented here were designed to investigate this hypothesis. The results indeed show that most of the scintillating sources we observed with the VLBA have an extremely compact, core-dominated morphology, which has important implications for interpretations of the scintillation seen in these sources. In addition, these sources are potential candidates for inclusion in the International Celestial Reference Frame.

Milliarcsecond-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 = -30° (positions for 10 of the sources are south of δ = -60°) 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 declination.

The Australia Telescope Compact Array (ATCA) has been used in support of the VLBI Space Observatory Programme (VSOP) space VLBI mission, to monitor the total and polarised flux densities of 202 objects that make up 83% of the VSOP all-sky Survey of compact extragalactic radio sources south of δ = + 10°. The primary goal of the ATCA observations is to provide information on the total and polarised emission from the compact components in these sources, for correlation with parameters obtained from VSOP imaging observations. These data represent the first high resolution, long timescale flux density monitoring observations of a large number of southern compact radio sources. In the future, comparison of the ATCA and VSOP data will be used to investigate relativistic beaming models and identify similarities or differences between the major classes of extragalactic radio sources. As an illustration of the scientific value of the ATCA data we undertake a comparison of the properties of the gamma-ray loud and gamma-ray quiet AGN in the southern component of the VSOP Survey sample, finding that in a flat-spectrum sub-sample the gamma-ray loud AGN are more variable than the gamma-ray quiet AGN.

Flux density monitoring data at 2.3 and 8.4 GHz is presented for a sample of 33 southern hemisphere GPS sources, drawn from the 2.7 GHz Parkes survey. This monitoring data, together with VLBI monitoring data, shows that a small fraction of these sources, ∼10%, vary. Their variability falls into several categories: sources whose spectral classification is, at best, ephemeral on a timescale of years; sources with a stable GPS classification that vary, but retain their classification; and a small number of sources that exhibit interstellar scintillation, but that maintain a mean GPS spectrum. Existing data on GPS sources with higher frequency peaks, ≥3 GHz, reveals that many such sources vary. However, the majority of these sources possess a GPS spectrum only during outbursts, and hence should perhaps be classified as ephemeral GPS sources. In addition, significant levels of circular polarisation have been found in a number of GPS sources, both amongst the variables and non-variables. Remarkable amongst these is PKS 1519–273, which possesses strong and variable circular polarisation, and which exhibits IDV in all Stokes parameters.

We have measured annual cycles in the time scales of the rapid, intra-day variability of the total and circularly polarized flux density, of the unusual BL Lac source PKS 1519-273 at 4.8 and 8.6 GHz. The data were collected at the ATCA over the last seven years, and establish unequivocally interstellar scintillation as the principal mechanism responsible for this cm-wavelength intra-day variability.

We report the discovery of H2O maser emission at 1.35 cm wavelength in seven active galactic nuclei (at distances up to

We present images from VLBI Space Observatory Programme (VSOP) observations of 14 compact extragalactic southern hemisphere radio sources, including a description of the observations, the data reduction techniques, and the parameters of the resulting images and model fits. These images provide the highest resolution information to date for many of these objects. Comparisons are made between VSOP and previous ground-based VLBI results, including images from data extracted from the geodetic VLBI archive at the United States Naval Observatory. From the VSOP data, we find that the two radio galaxies observed have lower peak brightness temperatures than the 12 quasars. Also, these data show (1) no evidence for obvious differences between the brightness temperature distributions of gamma-ray-loud and gamma-ray-quiet radio-loud active galactic nuclei and (2) no evidence for obvious correlations between brightness temperature and spectral index, radio polarization, flux density, or month timescale modulation index. These results are consistent with previous work by Lister, Tingay, & Preston, who found that the only observable significantly correlated with VSOP-derived brightness temperature is intraday variability, which is strongly correlated with many relativistic beaming indicators. For one source, PKS 1127-145, we undertake a detailed investigation of the milliarcsecond-scale component positions as a function of time, taking data from the literature and the current work, to estimate proper motions. As a result, we suggest that two components previously reported as stationary, C1 and C2, have apparent transverse speeds of (9.1 ± 3.8) and (5.3 ± 2.3) h-1c, respectively. We also make the first investigation of the apparent motion in the nearest GHz-peaked spectrum radio galaxy, PKS 1718-649, finding an upper limit on the apparent separation speed of 0.08c. Comparison of geodetic VLBI and VSOP data show no significant detection of component motion in PKS 0208-512, (2.4 ± 3.1) h-1c, and only a tentative detection in PKS 0537-441, (2.8 ± 2.2) h-1c. A significant detection of component motion is found in PKS 1610-771, solely from the geodetic VLBI data, (9.4 ± 3.5) h-1c.