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Gereb K.,Eötvös Loránd University | Gereb K.,University of Groningen | Frey S.,FOMI Satellite Geodetic Observatory | Frey S.,MTA Research Group for Physical Geodesy and Geodynamics
Advances in Space Research | Year: 2011

The point source list of the Wilkinson Microwave Anisotropy Probe (WMAP) is a uniform, all-sky catalogue of bright sources with flux density measurements at high (up to 94 GHz) radio frequencies. We investigated the five-year WMAP list to compile a new catalogue of bright and compact extragalactic radio sources to be potentially studied with Very Long Baseline Interferometry at millimeter wavelengths (mm-VLBI) and Space VLBI (SVLBI). After comparing the WMAP data with the existing mm-VLBI catalogues, we sorted out the yet unexplored sources. Using the 41, 61 and 94 GHz WMAP flux densities, we calculated the spectral indices. By collecting optical identifications, lower-frequency radio flux densities and VLBI images from the literature, we created a list of objects which have not been investigated with VLBI at 86 GHz before. With total flux density at least 1 Jy and declination above -40°, we found 37 suitable new targets. It is a nearly 25% addition to the known mm-VLBI sources. Such objects are also potentially useful as phase-reference calibrators for the future Japanese SVLBI mission ASTRO-G at its highest observing frequency (43 GHz). The phase-referencing capability of ASTRO-G would allow long integrations and hence better sensitivity for observing faint target sources close to suitable phase calibrators in the sky. © 2011 COSPAR. Published by Elsevier Ltd. All rights reserved. 18.


Moor A.,FMI Satellite Geodetic Observatory | Moor A.,MTA Research Group for Physical Geodesy and Geodynamics | Moor A.,Hungarian Academy of Sciences | Frey S.,FMI Satellite Geodetic Observatory | And 6 more authors.
Astronomical Journal | Year: 2011

Many of the compact extragalactic radio sources that are used as fiducial points to define the celestial reference frame are known to have proper motions detectable with long-term geodetic/astrometric very long baseline interferometry (VLBI) measurements. These changes can be as high as several hundred microarcseconds per year for certain objects. When imaged with VLBI at milliarcsecond (mas) angular resolution, these sources (radio-loud active galactic nuclei) typically show structures dominated by a compact, often unresolved "core" and a one-sided "jet." The positional instability of compact radio sources is believed to be connected with changes in their brightness distribution structure. For the first time, we test this assumption in a statistical sense on a large sample rather than on only individual objects. We investigate a sample of 62 radio sources for which reliable long-term time series of astrometric positions as well as detailed 8 GHz VLBI brightness distribution models are available. We compare the characteristic direction of their extended jet structure and the direction of their apparent proper motion. We present our data and analysis method, and conclude that there is indeed a correlation between the two characteristic directions. However, there are cases where the 1-10mas scale VLBI jet directions are significantly misaligned with respect to the apparent proper motion direction. © 2011. The American Astronomical Society. All rights reserved.


Rushton A.,European Southern Observatory | Rushton A.,Onsala Space Observatory | Miller-Jones J.C.A.,Curtin University Australia | Campana R.,Istituto di Astrofisica Spaziale e Fisica Cosmica | And 11 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We present evidence for the presence of a weak compact jet during a soft X-ray state of Cygnus X-1. Very-high-resolution radio observations were taken with the VLBA, EVN and MERLIN during a hard-to-soft spectral state change, showing the hard state jet to be suppressed by a factor of about 3-5 in radio flux and unresolved to direct imaging observations (i.e. ≲1 mas at 4cm). High time-resolution X-ray observations with the RXTE-PCA were also taken during the radio monitoring period, showing the source to make the transition from the hard state to a softer state (via an intermediate state), although the source may never have reached the canonical soft state. Using astrometric very long baseline interferometry (VLBI) analysis and removing proper motion, parallax and orbital motion signatures, the residual positions show a scatter of ∼0.2 mas (at 4cm) and ∼3 mas (at 13cm) along the position angle of the known jet axis; these residuals suggest that there is a weak unresolved outflow, with varying size or opacity, during intermediate and soft X-ray states. Furthermore, no evidence was found for extended knots or shocks forming within the jet during the state transition, suggesting that the change in outflow rate may not be sufficiently high to produce superluminal knots. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Cseh D.,University Paris Diderot | Corbel S.,University Paris Diderot | Kaaret P.,University of Iowa | Lang C.,University of Iowa | And 6 more authors.
Astrophysical Journal | Year: 2012

We present new radio, optical, and X-ray observations of three ultraluminous X-ray sources (ULXs) that are associated with large-scale nebulae. We report the discovery of a radio nebula associated with the ULX IC 342 X-1 using the Very Large Array (VLA). Complementary VLA observations of the nebula around Holmberg II X-1, and high-frequency Australia Telescope Compact Array and Very Large Telescope spectroscopic observations of NGC 5408 X-1 are also presented. We study the morphology, ionization processes, and the energetics of the optical/radio nebulae of IC 342 X-1, Holmberg II X-1, and NGC 5408 X-1. The energetics of the optical nebula of IC 342 X-1 is discussed in the framework of standard bubble theory. The total energy content of the optical nebula is 6 × 1052erg. The minimum energy needed to supply the associated radio nebula is 9.2 × 1050erg. In addition, we detected an unresolved radio source at the location of IC 342 X-1 at the VLA scales. However, our Very Long Baseline Interferometry (VLBI) observations using the European VLBI Network likely rule out the presence of any compact radio source at milliarcsecond (mas) scales. Using a simultaneous Swift X-ray Telescope measurement, we estimate an upper limit on the mass of the black hole in IC 342 X-1 using the "fundamental plane" of accreting black holes and obtain M BH ≤ (1.0 0.3) × 103 M. Arguing that the nebula of IC 342 X-1 is possibly inflated by a jet, we estimate accretion rates and efficiencies for the jet of IC 342 X-1 and compare with sources like S26, SS433, and IC 10 X-1. © 2012 The American Astronomical Society. All rights reserved.


Alexandroff R.,Princeton University | Overzier R.A.,University of Texas at Austin | Paragi Z.,Joint Institute for VLBI in Europe JIVE | Paragi Z.,MTA Research Group for Physical Geodesy and Geodynamics | And 7 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We have used the European very long baseline interferometry (VLBI) Network (EVN) to observe a sample of Lyman break analogues (LBAs), nearby (z < 0.3) galaxies with properties similar to the more distant Lyman break galaxies (LBGs). The study of LBGs may help define the feedback relationship between black holes (BHs) and their host galaxies. Previous Very Large Array (VLA) observations have shown that the kpc-scale radio emission from LBAs is dominated by starbursts. The main targets of this VLBI experiment were selected because they possessed emission-line properties between starbursts and Type 2 (obscured) active galactic nuclei (AGN). Eight targets (three star-forming LBAs, four composite LBAs and one Type 1 AGN) were observed at 5GHz, four of which (one star-forming LBA and three composite LBAs) were also observed at 1.7GHz. One star-forming LBA and one composite LBA were detected above 5σ at 1.7GHz (only), while the AGN was detected at 5GHz. In both LBAs, the radio luminosity (L R) exceeds that expected from supernovae (remnants) based on a comparison with Arp 220, Arp 229A and Mrk 273, by factors of. The composite LBA exhibits a compact core emitting around 10per cent of the VLA flux density. The high T b of 3.5 × 10 7K and excess core L R with respect to the L R/L X relation of radio-quiet AGN indicate that this LBA possesses an obscured AGN (M BH∼ 10 5 - 7M ⊙). In three other composite LBAs detected previously in the X-ray, no radio sources were detected, indicating either variability or the presence of an obscured AGN below our radio sensitivity. While weak AGN may coexist with the starbursts as shown in at least one of the LBAs, their contribution to the total radio flux is fairly minimal. Our results show that the detection of such weak AGN presents a challenge at radio, X-ray and optical emission-line wavelengths at z∼ 0.2, indicating the great difficulties that need to be overcome in order to study similar processes at high redshift when these types of galaxies were common. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Yang J.,Joint Institute for VLBI in Europe | Brocksopp C.,University College London | Corbel S.,University Paris Diderot | Paragi Z.,Joint Institute for VLBI in Europe | And 3 more authors.
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2010

The recently discovered Galactic X-ray transient XTE J1752-223 entered its first known outburst in 2010, emitting from the X-ray to the radio regimes. Its general X-ray properties were consistent with those of a black hole candidate in various spectral states, when ejection of jet components is expected. To verify this, we carried out very long baseline interferometry (VLBI) observations. The measurements were carried out with the European VLBI Network (EVN) and the Very Long Baseline Array (VLBA) at four epochs in 2010 February. The images at the first three epochs show a moving jet component that is significantly decelerated by the last epoch, when a new jet component appears that is likely to be associated with the receding jet side. The overall picture is consistent with an initially mildly relativistic jet, interacting with the interstellar medium or with swept-up material along the jet. The brightening of the receding ejecta at the final epoch can be well explained by initial Doppler deboosting of the emission in the decelerating jet. © 2010 The Authors. Journal compilation © 2010 RAS.


Yang J.,Joint Institute for VLBI in Europe | Wu F.,Chinese Academy of Sciences | Wu F.,University of Chinese Academy of Sciences | Paragi Z.,Joint Institute for VLBI in Europe | And 3 more authors.
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2012

The blueshifted broad absorption lines (BAL) or troughs are observed in active galactic nuclei (AGNs) when our line of sight is intercepted by a high-speed outflow (wind), likely originating in the accretion disc. The outflow or wind can shed light on the internal structure obscured by the AGN torus. Recently, it has been shown that this outflow is rotating in the BAL quasar PG 1700+518, further supporting the accretion disc origin of the wind. With the purpose of giving independent constraints on the wind geometry, we performed high-resolution European very long baseline interferometry (VLBI) Network (EVN) observations at 1.6GHz in 2010. Combining the results with the Very Large Array (VLA) archival data at 8.4GHz, we present its jet structure on scales of parsec (pc) to kiloparsec (kpc) for the first time. The source shows two distinct jet features in east-west direction with a separation of around 4kpc. The eastern feature, which has so far been assumed to hide the core, is a kpc-scale hotspot, which is completely resolved out in the EVN image. In the western jet feature, we find a compact jet component, which pinpoints the position of the central black hole in the galaxy. Jet components on both sides of the core are additionally detected in the north-west-south-east direction, and they show a symmetric morphology on scale of <1kpc. This two-sided jet feature is not common in the known BAL quasars and indicates that the jet axis is far away from the line of sight. Furthermore, it is nearly parallel to the scattering plane revealed earlier by optical polarimetry. By analogy to polar-scattered Seyfert1 galaxies, we conclude that the jet likely has a viewing angle around 45°. The analogy is further supported by the recent report of significant cold absorption in the soft X-rays, a nearly unique feature to polar-scattered Seyfert galaxies. Finally, our observations have confirmed the earlier finding that the majority of radio emission in this galaxy arises from AGN activity rather than star formation. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Yang J.,Joint Institute for VLBI in Europe | Paragi Z.,Joint Institute for VLBI in Europe | Paragi Z.,MTA Research Group for Physical Geodesy and Geodynamics | Corbel S.,University Paris Diderot | And 4 more authors.
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2011

The Galactic X-ray transient XTE J1752-223 was shown to have properties of black hole binary candidates. As reported in our previous paper, we identified transient and decelerating ejecta in multi-epoch Very Long Baseline Interferometry (VLBI) observations with the European VLBI Network (EVN) and the NRAO Very Long Baseline Array (VLBA). Here we present new EVN and VLBA data in which a new transient ejection event and later a stationary component are identified. The latter is interpreted as a reappearance of the radio core/compact jet during the transition from soft to hard X-ray state. This component appears to be highly variable in brightness although effects of tropospheric instabilities might play a role too. We also re-analyse the earlier VLBI data and find that the transient ejecta closer to the core position has significantly higher proper motion, further strengthening the case for strongly decelerating ejecta on the scale of several hundred milliarcsecond, never observed in X-ray binaries before. Although the distance of the source is not well constrained, it is clear that these ejectas are at least mildly relativistic at the early stages. Moreover, we show the large scale environment of the transient from the Westerbork synthesis array data recorded in parallel during the EVN run. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Frey S.,FOMI Satellite Geodetic Observatory | Frey S.,Joint Institute for VLBI in Europe | Paragi Z.,Joint Institute for VLBI in Europe | Paragi Z.,University Paris Diderot | And 5 more authors.
Astronomy and Astrophysics | Year: 2010

Context. Until now, there have only been seven quasars at z>4.5 whose the high-resolution radio structure had been studied in detail with Very Long Baseline Interferometry (VLBI) imaging. Aims. We almost double the number of VLBI-imaged quasars at these high redshifts with the aim of studying their redshift-dependent structural and physical properties in a larger sample. Methods. We observed five radio quasars (J0813+3508, J1146+4037, J1242+5422, J1611+0844, and J1659+2101) at 4.5 < z < 5 with the European VLBI Network (EVN) at 1.6 GHz on 29 October 2008 and at 5 GHz on 22 October 2008. The angular resolution achieved ranges from 1.5 to 25 milli-arcsec (mas), depending on the observing frequency, the position angle in the sky, and the source's celestial position. Results. The sources are all somewhat extended on mas scales, but compact enough to be detected at both frequencies. With one exception of a flat-spectrum source (J1611+0844), their compact emission is characterised by a steep radio spectrum. We found no evidence of Doppler-boosted radio emission in the quasars in our sample. The radio structure of one of them (J0813+3508) is extended to ∼7, which corresponds to 43 kpc projected linear size. Many of the highest redshift compact radio sources are likely to be young, evolving objects, far-away cousins of the powerful gigahertz peaked-spectrum (GPS) and compact steep-spectrum (CSS) sources that populate the Universe at lower redshifts. © 2010 ESO.


Frey S.,FOMI Satellite Geodetic Observatory | Frey S.,MTA Research Group for Physical Geodesy and Geodynamics | Paragi Z.,Joint Institute for VLBI in Europe | Paragi Z.,MTA Research Group for Physical Geodesy and Geodynamics | And 5 more authors.
Astronomy and Astrophysics | Year: 2011

Context. There are about 60 quasars known at redshifts z > 5.7 to date. Only three of them are detected in the radio above 1 mJy flux density at 1.4 GHz frequency. Among them, J1429+5447 (z = 6.21) is the highest-redshift radio quasar known at present. These rare, distant, and powerful objects provide important insight into the activity of the supermassive black holes in the Universe at early cosmological epochs and into the physical conditions of their environment. Aims. We studied the compact radio structure of J1429+5447 on the milli-arcsecond (mas) angular scale to compare the structural and spectral properties with those of another two z ~ 6 radio-loud quasars, J0836+0054 (z = 5.77) and J1427+3312 (z = 6.12). Methods. We performed Very Long Baseline Interferometry (VLBI) imaging observations of J1429+5447 with the European VLBI Network (EVN) at 1.6 GHz on 2010 June 8, and at 5 GHz on 2010 May 27. Results. Based on its observed radio properties, the compact but somewhat resolved structure on linear scales of <100 pc, and the steep spectrum, the quasar J1429+5447 is remarkably similar to J0836+0054 and J1427+3312. To answer the question whether the compact steep-spectrum radio emission is a "universal" feature of the most distant radio quasars, it is essential to study more yet to be discovered radio-loud active galactic nuclei at z > 6. © 2011 ESO.

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