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 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 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:
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FileName Lrecl Records Explanations
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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)
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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
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Bytes Format Units Label Explanations
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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)
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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 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 1995ApJ...450..559B, Cat. VIII/71
NVSS = position accuracy of about 1",
Condon et al., 1998AJ....115.1693C 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.
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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