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J/A+A/520/A113 VLBI detection of 398 extragalactic radio sources (Bourda+, 2010)

VLBI observations of optically-bright extragalactic radio sources for the alignment of the radio frame with the future Gaia frame. I. Source detection. Bourda G., Charlot P., Porcas R.W., Garrington S.T. <Astron. Astrophys. 520, A113 (2010)> =2010A&A...520A.113B
ADC_Keywords: VLBI ; Interferometry ; Radio sources ; QSOs ; Active gal. nuclei ; BL Lac objects Keywords: reference systems - quasars: general - astrometry - catalogs - methods: observational - techniques: interferometric Abstract: The European space astrometry mission Gaia will construct a dense optical QSO-based celestial reference frame. For consistency between optical and radio positions, it will be important to align the Gaia and VLBI frames with the highest possible accuracy. It has been found that only 70 (10%) of the sources from the International Celestial Reference Frame (ICRF) are suitable for establishing this link, 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 situation, we have initiated a VLBI survey dedicated to finding additional suitable radio sources for aligning the two frames. The sample consists of 447 optically- bright (magnitude ≤18) extragalactic radio sources, typically 20 times weaker than the ICRF sources, which have been selected by cross-correlating an optical quasar catalog with the NRAO VLA Sky Survey (NVSS, Cat. VIII/65). This paper presents the observing strategy to detect, image, and measure accurate radio positions for these sources. It also provides results on the VLBI detectability of the sources, as derived from initial observations with the European VLBI Network in June and October 2007. Based on these observations, a high detection rate of 89% is found, which is promising for the continuation of this project. This high VLBI detection rate for sources from the NVSS catalog is probably due to the selection process, suggesting that optically-bright quasars have compact radio structures. Description: Two 48-hour experiments were carried out in June and October 2007 (EC025A and EC025B, respectively), with the European VLBI Network (EVN) recording at 1Gbps in a geodetic-style dual-frequency S/X mode (2.3 and 8.4GHz), to determine the VLBI detectability of the 447 optically-bright weak extragalactic radio sources pre-selected for the alignment of the VLBI frame with the future Gaia frame. The list of the 398 extragalactic radio sources detected during these experiments is detailed in this table, as well as the apparent optical magnitude, the redshift and the type of the object from Veron-Cetty & Veron (2006A&A...455..773V, Cat. VII/248). The right ascension and declination are extracted from the catalog specified in the 3rd column. The mean correlated flux densities are given in mJy, the derived S/X spectral index is also specified, and the name of the experiment is given in the last column. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file table3.dat 105 398 List of the 398 extragalactic radio sources detected during EC025A and EC025B experiments (211 out of 218 detected during EC025A, and 187 out of 229 detected during EC025B) table4.dat 105 49 List of the 49 extragalactic radio sources not detected in one or both bands during EC025A and EC025B (7 sources out of 218 observed during EC025A and 42 sources out of 229 observed during EC025B)
See also: VIII/65 : 1.4GHz NRAO VLA Sky Survey (NVSS) (Condon+ 1998) VIII/71 : The FIRST Survey Catalog, Version 03Apr11 (Becker+ 2003) VIII/72 : CLASS survey of radio sources (Myers+, 2003) VII/248 : Quasars and Active Galactic Nuclei (12th Ed.) (Veron+ 2006) J/ApJS/171/61 : All-Sky Survey of Flat-Spectrum Radio Sources (Healey+, 2007) Byte-by-byte Description of file: table[34].dat
Bytes Format Units Label Explanations
1- 8 A8 --- IERS IERS designation (HHMM+DDd) 9 A1 --- n_IERS [*] Gravitational lenses (1) 12- 28 A17 --- Source Source name (2) 31- 36 A6 --- Cat Catalog (2) 39- 43 F5.2 mag Vmag Optical magnitude from Veron-Cetty and Veron, 2006, Cat. VII/248 46- 50 F5.3 --- z ? Redshift from Veron-Cetty and Veron, 2006, Cat. VII/248 53- 55 A3 --- Type [BL QUA] Type of the object: BL Lac or Quasar (from Veron-Cetty and Veron, Cat. VII/248) 58- 59 I2 h RAh Right ascension (J2000) 61- 62 I2 min RAm Right ascension (J2000) 64- 70 F7.4 s RAs Right ascension (J2000) 73 A1 --- DE- Declination sign (J2000) 74- 75 I2 deg DEd Declination (J2000) 77- 78 I2 arcmin DEm Declination (J2000) 80- 85 F6.3 arcsec DEs Declination (J2000) 88- 90 I3 mJy Ss ? S-band (2.3GHz) mean correlated flux density (3) 93- 95 I3 mJy Sx ? X-band (8.4GHz) mean correlated flux density (3) 98-102 F5.2 --- alpha ? S/X spectral index (only in table3.dat) 105 A1 --- EXP [A/B] Experiment when the source was observed: 'A' = EC025A (18-20 June 2007) 'B' = EC025B (26-28 October 2007)
Note (1): The 3 sources which name is marked with an asterisk are gravitational lenses which will be omitted from subsequent steps of the project. Note (2): The catalogs from which the coordinates were extracted are: CLASS = position accuracy of about 35mas, Myers et al., 2003MNRAS.341....1M, Cat. VIII/72 CRATES = position accuracy ranging from about 50mas to 1", Healey et al., 2007, Cat. J/ApJS/171/61 FIRST = position accuracy ranging from about 50mas to 1", Becker et al., 1995ApJ...450..559B, Cat. VIII/71 NVSS = position accuracy of about 1", Condon et al., 1998AJ....115.1693C, Cat. VIII/65 AHA = position accuracy better than 1", Andrei, 2007, Private communication of improved positions for QSOs as extracted from the input catalog for Gaia. The name of the source written in the table is actually extracted from Veron-Cetty and Veron, Cat. VII/248 Note (3): Blanks stands for a non-detection at this frequency.
Acknowledgements: Geraldine Bourda, Geraldine.Bourda(at)obs.u-bordeaux1.fr History: * 14-Oct-2010: table3 (from author) * 14-Dec-2010: table4 added (from electronic version); missing minus sign added for the 6 sources with declination between 0 and -1° (thanks H. Andernach)
(End) Geraldine Bourda [LAB, France], Patricia Vannier [CDS] 20-Jul-2010
The document above follows the rules of the Standard Description for Astronomical Catalogues.From this documentation it is possible to generate f77 program to load files into arrays or line by line

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