- ID:
- ivo://CDS.VizieR/J/A+A/526/A102
- Title:
- VLBI imaging of 105 extragalactic radio sources
- Short Name:
- J/A+A/526/A102
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The European space astrometry mission Gaia, to be launched by 2012, will construct a dense optical QSO-based celestial reference frame which will need to be linked to the International Celestial Reference Frame (ICRF; the IAU fundamental frame), with the highest accuracy. However, it has been found that only 10% of the ICRF sources (70 sources) are suitable to establish this link. The remaining sources are not useful either because they are not bright enough at optical wavelengths or because they have significant extended radio emission which precludes reaching the highest astrometric accuracy. In order to improve the accuracy of this alignment, we have developed a program of VLBI observations based on three steps to detect, image and measure astrometric positions of weak extragalactic radio sources, with bright optical counterparts, from a sample of 447 candidate sources. The experiments devoted to VLBI detection, carried out with the European VLBI Network (EVN) in June and October 2007, were very successful, with 398 sources detected at both S- and X-bands. From these, 105 sources were observed in March 2008 with a global VLBI array (EVN and VLBA; Very Long Baseline Array) for imaging their VLBI structures. All sources were successfully imaged in both bands and about 50% (47 sources) were found to be point-like on VLBI scales. These images are available at http://www.obs.u-bordeaux1.fr/BVID/GC030/. VLBI positions of these sources will be measured accurately in future astrometric experiments.
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Search Results
- ID:
- ivo://CDS.VizieR/J/ApJ/659/1040
- Title:
- VLBI monitoring of NGC 4258
- Short Name:
- J/ApJ/659/1040
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report a 3yr, 18 epoch, VLBI monitoring study of H_2_O masers in the subparsec, warped, accretion disk within the NGC 4258 AGN. Our immediate goals are to trace the geometry of the underlying disk, track rotation via measurement of proper motion, and ascertain the radii of masers for which centripetal acceleration may be measured separately. The monitoring includes 4 times as many epochs, 3 times denser sampling, and tighter control over sources of systematic error than earlier VLBI investigations. Coverage of a 2400km/s bandwidth has also enabled mapping of molecular material 30% closer to the black hole than accomplished previously, which will strengthen geometric and dynamical disk models. Through repeated observation we have also measured for the first time a 5uas (1sigma) thickness of the maser medium.
- ID:
- ivo://CDS.VizieR/J/ApJS/155/33
- Title:
- VSOP 5 GHz AGN Survey. III
- Short Name:
- J/ApJS/155/33
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The VLBI Space Observatory Programme (VSOP) mission is a Japanese-led project to study radio sources with sub-milliarcsec resolution using an orbiting 8m telescope, HALCA, along with global arrays of Earth-based telescopes. Approximately 25% of the observing time is devoted to a survey of compact active galactic nuclei (AGNs) that are stronger than 1Jy at 5GHz - the VSOP AGN Survey. This paper, the third in the series, presents the results from the analysis of the first 102 Survey sources.
- ID:
- ivo://CDS.VizieR/J/ApJ/616/110
- Title:
- VSOP 5GHz AGN Survey. IV
- Short Name:
- J/ApJ/616/110
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The VLBI Space Observatory Programme (VSOP) mission is a Japanese-led project to study radio sources with submilliarcsecond angular resolution, using an orbiting 8m telescope on board the satellite HALCA with a global Earth-based array of telescopes. A major program is the 5GHz VSOP Survey Program, which we supplement here with Very Long Baseline Array observations to produce a complete and flux density-limited sample. Using statistical methods of analysis of the observed visibility amplitude versus projected (u, v) spacing, we have determined the angular size and brightness temperature distribution of bright radio emission from active galactic nuclei.
- ID:
- ivo://CDS.VizieR/J/ApJS/175/314
- Title:
- VSOP 5GHz AGN survey V.
- Short Name:
- J/ApJS/175/314
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In 1997 February, the Japanese radio astronomy satellite HALCA was launched to provide the space-bourne element for the VLBI Space Observatory Program (VSOP) mission. Approximately 25% of the mission time was dedicated to the VSOP survey of bright compact active galactic nuclei (AGNs) at 5GHz. This paper, the fifth in the series, presents images and models for the remaining 140 sources not included in the third paper in the series, which contained 102 sources. For most sources, the plots of the (u,v) coverage, the visibility amplitude versus (u,v) 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.
- ID:
- ivo://CDS.VizieR/J/A+A/641/A162
- Title:
- Water maser data of 380 galaxies
- Short Name:
- J/A+A/641/A162
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Water megamaser emission at 22GHz has proven to be a powerful tool for astrophysical studies of active galactic nuclei (AGN) because it allows an accurate determination of the mass of the central black hole and of the accretion disc geometry and dynamics. However, after searches among thousands of galaxies, only ~200 of them have shown such spectroscopic features, most of them of uncertain classification. In addition, the physical and geometrical conditions under which a maser activates are still unknown. We characterize the occurrence of water maser emission in an unbiased sample of AGN by investigating the relation with the X-ray properties and the possible favourable geometry that is required to detect water maser. We searched for 22GHz maser emission in a hard X-ray selected sample of AGN, taken from the INTEGRAL/IBIS survey above 20keV. Only half of the 380 sources in the sample have water maser data. We also considered a volume-limited sub-sample of 87 sources, for which we obtained new observations with the Green Bank and Effelsberg telescopes (for 35 sources). We detected one new maser and increased its radio coverage to 75%. The detection rate of water maser emission in the total sample is 15+/-3%. This fraction increases to 19+/-5% for the complete sub-sample, especially when we consider type 2 (22+/-5% and 31+/-10% for the total and complete samples, respectively) and Compton-thick AGN (56+/-18% and 50+/-35% for the total and complete samples, respectively). No correlation is found between water maser and X-ray luminosity. We note that all types of masers (disc and jet) are associated with hard X-ray selected AGN. These results demonstrate that the hard X--ray selection may significantly enhance the maser detection efficiency over comparably large optical or infrared surveys. A possible decline in detection fraction with increasing luminosity might suggest that an extremely luminous nuclear environment does not favour maser emission. The large fraction of CT AGN with water maser emission could be explained in terms of geometrical effects. The maser medium would then be the very edge-on portion of the obscuring medium.
- ID:
- ivo://CDS.VizieR/J/ApJ/652/136
- Title:
- Water maser emission in five AGNs
- Short Name:
- J/ApJ/652/136
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report the discovery of water maser emission in five active galactic nuclei (AGNs) with the 100m Green Bank Telescope (GBT). 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" radio and 1.3" optical) and most likely mark the locations of the embedded central engines. The spectra of three sources, 2MASX J08362280+3327383, NGC 6264, and UGC 09618 NED02 (PGC 053432), display the characteristic spectral signature of emission from an edge-on accretion disk with maximum orbital velocity of ~700, ~800, and ~1300km/s, respectively. We also present a GBT spectrum of a previously known source, Mrk 0034, and interpret the narrow Doppler components reported here as indirect evidence that the emission originates in an edge-on accretion disk with orbital velocity of ~500km/s .
- ID:
- ivo://CDS.VizieR/J/MNRAS/400/984
- Title:
- WMAP 3-yr sources at 16 and 33GHz
- Short Name:
- J/MNRAS/400/984
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present follow-up observations of 97 point sources from the Wilkinson Microwave Anisotropy Probe (WMAP) 3-yr data, contained within the New Extragalactic WMAP Point Source catalogue between -4{deg}<=DE<=60{deg}; the sources form a flux-density-limited sample complete to 1.1Jy (~5{sigma}) at 33GHz. Our observations were made at 16GHz using the Arcminute Microkelvin Imager and at 33GHz with the Very Small Array (VSA).
- ID:
- ivo://CDS.VizieR/J/MNRAS/400/995
- Title:
- WMAP 3-yr sources at 16 and 33GHz. II.
- Short Name:
- J/MNRAS/400/995
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Using the Arcminute Microkelvin Imager (AMI) at 16GHz and the Very Small Array (VSA) at 33GHz to make follow-up observations of sources in the New Extragalactic WMAP Point Source catalogue, we have investigated the flux density variability in a complete sample of 97 sources over time-scales of a few months to ~1.5yr.
- ID:
- ivo://CDS.VizieR/J/A+A/471/1105
- Title:
- XMM-LSS at 240MHz and 610MHz
- Short Name:
- J/A+A/471/1105
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The low-frequency radio survey of the XMM-Large Scale Structure (XMM-LSS) field aims to study the connection between the extragalactic radio source populations and their environment as traced by X-ray and optical emission. In this paper we present new radio observations of the XMM-LSS field carried out using the Giant Meterwave Radio Telescope at 240 and 610MHz. These observations complement the observations presented by Cohen at al. (2003, Cat. <J/ApJ/591/640>) and Tasse et al. (2006, Cat. <J/A+A/456/791>) at 74 and 325MHz with the Very Large Array. At 240 and 610MHz, we reach noise levels of ~2.5 and ~0.3mJy/beam, leading to the detection of 466 and 769 sources over 18.0 and 12.7 degree^2^ with resolutions of 14.7arcsec and 6.5arcsec respectively. Combining these data with the available source lists at 74, 325 (Tasse et al., 2006, Cat. <J/A+A/456/791>) and 1400MHz (NVSS), we build a multifrequency catalogue containing 1611 radio sources. We check for consistency of the astrometry and flux density estimates. We fit a simple synchrotron radiation model to the flux density measurements of the 318 radio sources being detected in at least 4 bands. While ~26% of them show signature of spectral ageing, ~6% show self absorption.