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J/A+A/526/A102   VLBI imaging of 105 extragalactic radio sources (Bourda+, 2011)

VLBI observations of optically-bright extragalactic radio sources for the alignment of the radio frame with the future Gaia frame. II. Imaging candidate sources. Bourda G., Collioud A., Charlot P., Porcas R.W., Garrington S.T. <Astron. Astrophys. 526, A102 (2011)> =2011A&A...526A.102B
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, 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. Description: The global experiment GC030 was carried out in March 2008, during 48-hrs, with the European VLBI Network (EVN) and the Very Long Baseline Array (VLBA) recording at 512 Mbps in a geodetic-style dual-frequency S/X mode (2.3 and 8.4GHz). These VLBI observations were dedicated to imaging 105 weak extragalactic radio sources, with bright optical counterparts, detected during previous VLBI experiments (Bourda et al., 2010A&A...520A.113B). The list of 105 target sources is detailed in this table, as well as the total flux densities as integrated from the X- and S-band images (units in mJy), the corresponding S/X-spectral index, the continuous structure index at X- and S-bands, and the parameters of the naturally weighted VLBI images at both bands. These latter are: (i) the restoring beam, which is an elliptical Gaussian with FWHM (Full Width at Half Maximum) major axis a and minor axis b (units in mas), with major axis in position angle phi (measured north through east; units in degrees); (ii) the peak brightness and the rms (root mean square) of the residuals of the final hybrid image (units in mJy/beam); (iii) the first contour level of the image (i.e. percentage of the peak and corresponding value in mJy/beam). File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file table3.dat 72 210 List of the 105 optically-bright, weak extragalactic radio sources, observed during GC030 at X- and S-bands
See also: J/A+A/520/A113 : 398 extragalactic radio sources VLBI detection (Bourda+, 2010) http://www.obs.u-bordeaux1.fr/BVID/GC030/ : Bordeaux VLBI Image Database Byte-by-byte Description of file: table3.dat
Bytes Format Units Label Explanations
1- 8 A8 --- IERS IERS designation (HHMM+DDd) 11 A1 --- Band [XS] Band of observation: X=8.4GHz, S=2.3GHz 14- 18 F5.1 mJy Flux Total flux density integrated from the image 21- 24 F4.1 --- alpha S/X spectral index αSX 27- 29 F3.1 --- SI Continuous structure index (1) 32- 35 F4.2 mas a Beam major axis (2) 38- 41 F4.2 mas b Beam minor axis (2) 44- 48 F5.1 deg phi Beam major axis position angle (2) 51- 55 F5.1 mJy Peak Peak brightness (in mJy/beam) 58- 61 F4.2 mJy RMS RMS of the residuals of the final hybrid image (in mJy/beam) 64- 66 F3.1 % FCL% First contour level: percentage of the peak 69- 72 F4.2 mJy FCL First contour level: value (in mJy/beam)
Note (1): the structure index is defined as SI = 1 + log(τm), where τm represents the median value of the structure delay corrections in picoseconds (ps=10-12s). Only sources with values SI<3.0 should be used for the alignment of the VLBI and Gaia frames. Note (2): The restoring beam is an elliptical Gaussian with FWHM (Full Width at Half Maximum) major axis a and minor axis b (units in mas), with major axis in position angle phi (measured North through East; units in degrees).
Acknowledgements: Geraldine Bourda, Geraldine.Bourda(at)obs.u-bordeaux1.fr References: Bourda et al., Paper I 2010A&A...520A.113B, Cat. J/A+A/520/A113
(End) Geraldine Bourda [LAB, France], Patricia Vannier [CDS] 23-Nov-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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