J/AJ/151/154 Second epoch VLBA Calibrator Survey (VCS-II) (Gordon+, 2016)
Second epoch VLBA Calibrator Survey observations: VCS-II.
Gordon D., Jacobs C., Beasley A., Peck A., Gaume R., Charlot P., Fey A.,
Ma C., Titov O., Boboltz D.
<Astron. J., 151, 154-154 (2016)>
=2016AJ....151..154G 2016AJ....151..154G (SIMBAD/NED BibCode)
ADC_Keywords: Surveys ; VLBI ; Radio sources ; QSOs
Keywords: astrometry - quasars: general - radio continuum: galaxies -
reference systems - surveys - techniques: interferometric
Abstract:
Six very successful Very Long Baseline Array (VLBA) calibrator survey
campaigns were run between 1994 and 2007 to build up a large list of
compact radio sources with positions precise enough for use as very
long baseline interferometry (VLBI) phase reference calibrators. We
report on the results of a second epoch VLBA Calibrator Survey
campaign (VCS-II) in which 2400 VCS sources were re-observed in the X
and S bands in order to improve the upcoming third realization of the
International Celestial Reference Frame (ICRF3) as well as to improve
their usefulness as VLBI phase reference calibrators. In this survey,
some 2062 previously detected sources and 324 previously undetected
sources were detected and revised positions are presented. Average
position uncertainties for the re-observed sources were reduced from
1.14 and 1.98mas to 0.24 and 0.41mas in R.A. and decl., respectively,
or by nearly a factor of 5. Minimum detected flux values were
approximately 15 and 28mJy in the X and S bands, respectively, and
median total fluxes are approximately 230 and 280mJy. The vast
majority of these sources are flat-spectrum sources, with ∼82% having
spectral indices greater than -0.5.
Description:
Six Very Long Baseline Array (VLBA) calibrator survey campaigns were
run between 1994 and 2007 (VCS1, Beasley et al. 2002, cat.
J/ApJS/141/13; VCS2, Fomalont et al. 2003, cat. J/AJ/126/2562; VCS3,
Petrov et al. 2005, cat. J/AJ/129/1163; VCS4, Petrov et al. 2006, cat.
J/AJ/131/1872; VCS5, Kovalev et al. 2007, cat. J/AJ/133/1236; VCS6,
Petrov et al. 2008, cat. J/AJ/136/580)
We report on the results of a second epoch VLBA Calibrator Survey
campaign (VCS-II) in which 2400 VCS sources were re-observed in the X
and S bands.
The VLBA S/X (S band∼2.3GHz and X band∼8.6GHz) dual frequency system
was used. We used the VLBA RDBE/Mark5C system, which has 16 32MHz
channels and records 2 Gbits/s using 2 bit sampling. Due to S-band
filters below 2200MHz and above 2400MHz at most of the VLBA antennas,
and a broad area of RFI from SiriusXM satellites (2320-2345MHz), only
four channels could be deployed in the S band (2220.0, 2252.0, 2284.0,
and 2348.0MHz). The other 12 channels were deployed in the X band
(8460.0, 8492.0, 8524.0, 8556.0, 8620.0, 8652.0, 8716.0, 8748.0,
8812.0, 8844.0, 8876.0, and 8908.0MHz).
We set a target of 300 sources per session, or 2400 total sources for
the 8 VLBA sessions. We selected all sources from the Goddard Space
Flight Center (GSFC) S/X astrometric/geodetic catalog (available at
http://gemini.gsfc.nasa.gov/solutions/ or by following the links at
http://lupus.gsfc.nasa.gov/) between -50° and +90° decl. that
had been observed in only 1 or 2 sessions as of mid 2013. This
amounted to ∼2060 sources. To fill out the list, we added ∼340
additional sources that had been observed but not detected in the
original VCS1-6 analysis.
The eight schedules were run between 2014 January and 2015 March
(VCS-II-A/BG219A on 2014 01/04 10:04-01/05 10:02; VCS-II-B/BG219B1 on
2014 05/31 17:12-06/01 17:05; VCS-II-D/BG219D on 2014 06/09
09:13-06/10 09:10; VCS-II-C/BG219C on 2014 08/05 13:03-08/06 13:00;
VCS-II-E/BG219E on 2014 08/09 00:00-08/09 23:55; VCS-II-F/BG219F on
2014 12/20 01:18-12/21 01:14; VCS-II-H/BG219H on 2015 01/23
23:00-01/24 22:55; VCS-II-I/BG219I on 2015 03/17 07:57-03/18 07:57).
Positions and other information for the 2062 re-observed and the 324
new sources are given in Tables 2 and 3.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 142 2062 VCS-II catalog, re-observed sources
table3.dat 142 324 VCS-II catalog, originally undetected VCS sources
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See also:
I/323 : International Celestial Reference Frame 2, ICRF2 (Ma+, 2009)
J/AJ/150/58 : VLBI ICRF2 (Fey+, 2015)
J/AJ/136/580 : Sixth VLBA calibrator survey: VCS6 (Petrov+, 2008)
J/AJ/133/1236 : Fifth VLBA calibrator survey: VCS5 (Kovalev+, 2007)
J/AJ/131/1872 : Fourth VLBA calibrator survey: VCS4 (Petrov+, 2006)
J/AJ/129/1163 : Third VLBA calibrator survey: VCS3 (Petrov+, 2005)
J/AJ/126/2562 : Second VLBA calibrator survey: VCS2 (Fomalont+, 2003)
J/ApJS/141/13 : VLBA calibrator survey: VCS1 catalog (Beasley+, 2002)
http://gemini.gsfc.nasa.gov/solutions/ : Full GSFC catalog of all S/X sources
http://lupus.gsfc.nasa.gov/ : GSFC
Byte-by-byte Description of file: table[23].dat
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Bytes Format Units Label Explanations
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1- 10 A10 --- VCS-II Identifier based on J2000 positions
(JHHMM+DDMM format) (1)
12- 19 A8 --- IVS Identifier from the International Very long
baseline interferometry (VLBI) Service for
Geodesy and Astrometry (IVS) (HHMM+DDd, B1950)
21- 22 I2 h RAh Hour of Right Ascension (J2000) (2)
24- 25 I2 min RAm Minute of Right Ascension (J2000) (2)
27- 36 F10.7 s RAs Second of Right Ascension (J2000) (2)
38 A1 --- DE- Sign of the Declination (J2000) (2)
39- 40 I2 deg DEd Degree of Declination (J2000) (2)
42- 43 I2 arcmin DEm Arcminute of Declination (J2000) (2)
45- 53 F9.6 arcsec DEs Arcsecond of Declination (J2000) (2)
55- 60 F6.3 mas e_RAs Inflated error in RAs (2)
62- 67 F6.3 mas e_DEs Inflated error in DEs (2)
69- 73 F5.3 Jy SStot [0.03/5.5] Total S band (∼2.3GHz) flux
density (3)
75- 79 F5.3 Jy SSun [0/2.36] Unresolved S band (∼2.3GHz) flux
density (3)
81- 85 F5.3 Jy SXtot [0.02/5.5] Total X band (∼8.6GHz) flux
density (3)
87- 91 F5.3 Jy SXun [0/1.57] Unresolved X band (∼8.6 GHz) flux
density (3)
93- 97 F5.2 --- alpha [-1.91/1.44] Spectral index α
(where Sν∝να) (3)
99-109 A11 "Y:M:D" Date Date of the flux observations
111-113 I3 --- Nobs [3/986] Total number of observations (4)
115-142 A28 --- OName Other name(s)
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Note (1): Of the 2400 target sources observed, 2386 were detected. To be
considered a "detection," we require at least 3 good S/X observation pairs,
with the fourfit program requiring an S/N of at least 7 for an observation
to be considered good. Of the 2386 sources detected, 2062 are previously
detected sources, and 324 are sources that were not detected in the
original VCS1-6 analysis.
Note (2): The positions and inflated errors are weighted averages using the
observations from the VCS-II sessions, the VCS1-6 sessions, and a few other
sessions for a few sources; except that for the three problematic sources
discussed in the previous paragraph, source data from the earlier VCS1-6
sessions was not used. These positions are extracted from the larger
Calc/Solve least square solution described in Section 3.
Note (3): Fluxes and spectral indices are computed from the VCS-II sessions
only. The spectral indices are estimated from the "total" fluxes in the S
and X bands, and are given to aid in estimating fluxes at other
frequencies.
Note (4): Observations (group delays) on the source used in the solution,
including earlier measurements.
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History:
From electronic version of the journal
(End) Prepared by [AAS]; Sylvain Guehenneux [CDS] 25-Jul-2016