The Second VLBA Calibrator Survey: VCS2

Abstract

This paper presents an extension of the Very Long Baseline Array Calibrator Survey, called VCS2, containing 276 sources. This survey fills in regions of the sky that were not completely covered by the previous VCS1 calibrator survey. The VCS2 survey includes calibrator sources near the Galactic plane, À30 < < À45 , and VLA calibrators. The positions have been derived from astrometric analysis of the group delays measured at 2.3 and 8.4 GHz using the Goddard Space Flight Center CALC/SOLVE package. From the VLBA snapshot observations, images of the calibrators are available, and each source is given a quality code for anticipated use. The VCS2 catalog is available from the NRAO Web site.

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THE SECOND VLBA CALIBRATOR SURVEY: VCS2
E. B.Fomalont
NationalRadioAstronomy Observatory,520EdgemontRoad,Charlottesville,VA22903;
efomalon@nrao.edu
L. Petrov and D. S. MacMillan
NVI,Inc.,NASAGoddardSpaceFlightCenter,Greenbelt,MD20771;
Leonid.Petrov@gsfc.nasa.gov,dsm@leo.gsfc.nasa.gov
D. Gordon
RaytheonInformationTechnology andInformationServicesandNASAGoddardSpaceFlightCenter,
Code926, Greenbelt, MD20771;
dgg@leo.gsfc.nasa.gov
and
C.Ma
NASAGoddard SpaceFlightCenter,Code926,Greenbelt,MD20771;
Chopo.Ma@nasa.gov
Received2003June2;accepted2003July 30
ABSTRACT
This paper presents an extension of the Very Long Baseline Array Calibrator Survey, called VCS2,
containing276sources.Thissurvey fillsinregionsoftheskythatwerenotcompletelycoveredbytheprevious
VCS1 calibrator survey. The VCS2 survey includes calibrator sources near the Galactic plane,
?30?< ? < ?45?, and VLA calibrators. The positions have been derived from astrometric analysis of the
group delays measured at 2.3 and 8.4 GHz using the Goddard Space Flight Center CALC/SOLVE package.
From the VLBA snapshot observations, images of the calibrators are available, and each source is given a
quality code for anticipated use. The VCS2 catalog is available from the NRAO Web site.
Key words:astrometry — catalogs — quasars: general — radio continuum — reference systems —
techniques:interferometric
On-line material:machine-readable table
1. INTRODUCTION
The Very Long Baseline Array (VLBA) Calibrator
Survey produced the VCS1 catalog, which contained 1332
radio sources, and is described by Beasley et al. 2002. Most
of these calibrator sources are suitable for phase referencing
VLBI observations (Beasley & Conway 1995), and images
for many of these sources are also available on-line. The
radio positions were derived on the frame work of the
International Celestial Reference System (ICRS) and
the International Terrestrial Reference System (ITRS). The
current realization of the ICRS and ITRS are the Inter-
national Celestial Reference Frame (ICRF) and the
International Terrestrial Reference Frame (ITRF 2000; Ma
et al. 1998; Altamimi, Sillard, & Boucher 2002). In this
paper we present a supplement to the VCS1 catalog, which
includes sources near the Galactic plane,1at declinations
between ?30?and ?45?, and from the VLA catalog of
sources. This extension is called the VCS2 catalog. Since the
observations, calibrations, astrometric solutions, and imag-
ing are virtually identical to that of the VCS1, most of the
details have been described in Beasley et al. 2002. In x 2 we
describe the compilation of the VCS2 source list, in x 3 we
list the catalog results, and in x 4 we summarize the results.
Unlike the VCS1 observations, many of the radio sources in
the VCS2 observing source list are weak, with unknown
source structure. Hence, only 70% are usable as VLBA
calibrators, and a quality code is attached to each VCS2
calibrator.
2. SOURCE SELECTION, OBSERVATIONS
AND CALIBRATION
The ICRF and VCS1 catalogs cover the sky in a relatively
uniform manner; however, two regions are relatively
sparsely sampled: declinations south of ?30?and the
Galactic plane. The major goal of the VCS2 observations
was to fill these regions with suitable calibrators. For the
declination band ?30?to ?45?, we used the preliminary
results from a VLA survey to locate gravitational lenses
(J. N. Winn & J. E. Lovell 2002, private communication).
We limited the VCS2 candidate sources from this survey to
those with flat radio spectra and stronger than 80 mJy. In
the declination range 0?to ?30?, we also included the less
frequently observed ICRF-Ext. 1 source list. For candidate
sources near the Galactic plane, we used primarily the J-net
Galactic Plane Survey for VLBI Exploration of Radio
Astronomy (Honma et al. 2000). Finally, we searched the
VLA calibrator source list and included those sources that
were flat spectrum and unresolved, with no previous VLBI
observations. The generated candidate source list for the
VCS2 observations contained 276 sources.
The VCS2 observations were carried out in two 24 hr
sessions: 2002 January 31 and 2002 May 14. The observ-
ations used the VLBA dual-frequency geodetic mode,
1Observations of sources within about 10?of the Galactic plane are
often affected by Galactic scattering that makes their apparent angular size
too large to be useful as calibrators for baselines longer than about
2000km.
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#2003.TheAmericanAstronomicalSociety.All rightsreserved.Printedin U.S.A.
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observing simultaneously at 2.3 GHz (S band) and 8.4 GHz
(X band). The data for each of these two frequencies were
separated into four 8 MHz channels, which spanned 140
MHz at 2.3 GHz and 490 MHz at 8.4 GHz, in order to
provide precise measurements of the group delays for
astrometricprocessing.
Nominally, each source was observed twice each for
about 90 s. For sources north of ?33?, the two observations
were made on either of the two observing days and sepa-
rated by at least 1 hr in order to improve the (u, v) coverage.
More southern sources were observed once on both days,
near transit. Because of the nonuniform distribution of the
sources around the sky and some data editing, a few sources
were observed three times and a few only one time. In addi-
tion to the targeted sources, three strong sources from a list
of 32 sources taken from the ICRF catalog were observed
every hour over the sky in order to allow estimation of the
tropospheric path delay and to evaluate possible systematic
errors of the VCS2 catalog.
The amplitude and initial phase calibration of the
observations was made using the Astronomical Image
Processing System (AIPS; Greisen 1988). Fringe-fitting and
determination of the group delays for each observation was
made by the Goddard Space Flight Center (GSFC) VLBI
group, also using AIPS. The software package CALC/
SOLVE for the astrometric analysis was then used to
determine accurate estimates of the position for all of the
candidate sources. Estimates of the temporal changes in the
clocks, the troposphere path delays, and other geodetic
information were also determined. The ionospheric delays
were removed by the appropriate difference of the measured
group delays at the two frequencies. For some of the weaker
and resolved sources, only 8.4 GHz data was used to
determine the source position. Errors of all parameters were
also determined. See Beasley et al. 2002 for more details
concerning the astrometric methods usedfor the solutions.
Comparison of common source positions of the VCS2
catalogandtheICRF Ext. 1catalogrevealed significantsys-
tematic errors of the VCS2 catalog. These errors increase
with decreasing declination and are given in Tables 1 and 2.
We believe these errors are caused by unmodeled variations
of the troposphere path delay, which become more signifi-
cant at low elevations, and preferentially for sources in the
south. We used these tables of errors for computation of
the source position uncertainties:
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi
??¼
where ?r? and ?r
E?(?) and E?(?) are the declination-dependent additive re-
??¼ð1:5?r?Þ2þ E2
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
?stand for the formal uncertainties,
?ð?Þ=cos2?
q
q
;
ð1:5?r
?Þ2þ E2
?ð?Þ
;
ð1Þ
weighting parameters for right ascension and declination,
respectively, taken from Tables 1or 2.
The data at each frequency were imaged at NRAO using
the Caltech package DIFMAP (Shepherd 1997). Starting
with a point-source model, progressively better images were
obtained for each source and frequency using the deconvo-
lution, editing, and self-calibration algorithms in DIFMAP.
Since many of the sources were relatively weak, natural
weighting,withsome loss ofresolution,was usedto produce
the images. Some sources were so weak and resolved that
the images were dominated by noise. The faintest
unresolved component that could be detected was ?0.07 Jy
at both frequencies.
During the source position determination at GSFC and
the source mapping at NRAO, it was clear that a significant
minority of the sources would not make good calibrators
for VLBI phase-referencing experiments. Hence, we decided
toinclude aqualitycodeatbothfrequenciesforeachsource,
and these are listed in the VCS2 catalog given below. These
quality codes are somewhat subjective, and the use of any
calibrator depends on the sensitivity, resolution, and fre-
quency of the VLBI observations. The amplitude versus
projected spacing properties of a source provide the best
information for the appropriate choice of calibrators.
3. THE VCS2 CATALOG
Asample of the VCS2 catalog islisted in Table3.The first
four columns give the source names and the position at
epoch J2000.0, with estimated 1 ? errors as described above.
Column (5) gives the number of baseline observations used
to derive the position. For a source observed two times
using all 45 VLBA baselines, 90 observations would be
made. Column (6) gives the solution type: X/S means that
an ionosphere free linear combination of the X-band and
S-band data was used for the position determination; X+
and X-o mean that only X-band group delays were used due
to the poor quality of the S-band data (X+) or because no
fringes at the S-band were detected (X-o). Column (7) gives
the average correlated flux density at 2.3 GHz for the source
at the shorter VLBA spacings, and column (8) gives the
quality. Columns (9) and (10) are similar to columns (7) and
(8), except at 8.4 GHz.
The calibrator qualities are indicative of their strength
and resolution at VLBA baselines. Those sources with a
quality factor of 5 should not be used. They are large-
diameter and weak sources and are included in the list for
completeness. The quality factors have the following
meaning:
1: Excellent quality.— Little change of signal over all
baselines andstronger than 0.4 Jy.
TABLE 1
Errors of the VCS2 X/S Positions
Decl.Zone
(deg)
D?* cos ?
(mas)
D?
(mas)No.ofPts.
(+90, +20)..............
(+20, ?30)..............
(?30, ?46)..............
0.3
0.3
0.6
0.3
0.6
1.1
12
15
11
TABLE 2
Errors of the VCS2 X-Band–Only Positions
Decl.Zone
(deg)
D?* cos ?
(mas)
D?
(mas)No.ofPts.
(+90, +20)..............
(+20, ?20)..............
(?20, ?30)..............
(?30, ?40)..............
(?40, ?46)..............
1.9
1.3
1.0
2.9
4.7
1.1
1.9
6.2
8.2
9.3
49
29
15
79
9
VLBA CALIBRATOR SURVEY2563

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2: Good quality.—Some decrease of signal at shorter
baselines, but stronger than 0.2 Jy at all baselines.
3: Average quality.—Decrease of signal at the shorter
baselines, but stronger than 0.10 Jy everywhere.
4: Fairquality.—Less than 0.10Jy onthelonger baselines,
but more signal on the shorter baselines. These calibrators
should be checked beforeusing.
5: Poor quality.—Less than 0.10 Jy on most baselines.
These calibrators should not be used. The positions could be
even more in error than indicated. Most sources within
about 10?of the Galactic plane are significantly broadened
by interstellar scattering at2.3GHz.
The complete VCS2 catalog is available on the World
Wide Web.2Contour plots of the images and of the visibility
amplitude versus projected spacing can also be found on the
VLBA calibrator Websearch page.3
Examples of the correlated flux density and image of
the sources with different qualities are shown in Figure 1.
Each line shows the 2.3 and 8.4 GHz visibility amplitude
and contour representation of the image. The sources
TABLE 3
The VCS2 Source List
SourceName
2.3GHz
8.4GHz
IAU
(1)
IVS
(2)
R.A.
(J2000.0)
(3)
Decl.
(J2000.0)
(4)
No.
(5)
SOL
(6)
S2.3
(7)
Q
(8)
S8.4
(9)
Q
(10)
J0000?3221 ..............
J0001+6051..............
J0004+2019..............
J0005?3445 ..............
J0006?2955 ..............
J0008+1144..............
J0009+0628..............
J0010+2047..............
J0010?3054 ..............
J0010?3027 ..............
J0010?4153 ..............
J0012?3954 ..............
J0014+6117..............
J0019?3031 ..............
J0024+2439..............
J0024?4202 ..............
J0025?2602 ..............
J0026?3512 ..............
J0027+0929..............
J0031?3922 ..............
J0036+1434..............
J0040?3243 ..............
J0040?3225 ..............
J0045?3900 ..............
J0049?3116 ..............
J0051?4226 ..............
J0058?3234 ..............
J0058?3347 ..............
J0059+5812..............
J0100?3337 ..............
J0109+6133..............
J0110+5632..............
J0119+0829..............
J0120?2701 ..............
J0124?3416 ..............
J0134?3843 ..............
J0134?0931 ..............
J0140+6346..............
J0143?3200 ..............
J0144?3938 ..............
J0151?3435 ..............
J0153?3310 ..............
J0155?4048 ..............
Notes.—Table 3 is presented in its entirety in the electronic edition of the Astronomical Journal. A portion is shown here for guidance regarding its
formandcontent.Unitsofrightascensionare hours,minutes,andseconds, andunitsofdeclinationare degrees,arcminutes,andarcseconds.
2357?326
2358+605
0002+200
0002?350
0003?302
0005+114
0006+061
0007+205
0008?311
0008?307
0008?421
0010?401
0012+610
0017?307
0021+243
0022?423
0023?263
0023?354
0024+092
0028?396
0033+142
0037?329
0038?326
0043?392
0046?315
0048?427
0055?328
0055?340
0056+579
0057?338
0106+612
0107+562
0116+082
0118?272
0122?345
0132?389
0132?097
0137+635
0140?322
0142?398
0149?348
0150?334
0153?410
000020.39995 ? 0.00007
000107.09961 ? 0.00064
000435.75831 ? 0.00003
000505.92510 ? 0.00030
000601.12325 ? 0.00023
000800.83833 ? 0.00006
000903.93187 ? 0.00002
001028.74312 ? 0.00016
001034.91052 ? 0.00123
001035.74240 ? 0.00018
001052.51774 ? 0.00033
001259.90986 ? 0.00007
001448.79215 ? 0.00022
001942.67533 ? 0.00006
002427.33055 ? 0.00003
002442.98922 ? 0.00023
002549.15625 ? 0.00008
002616.38710 ? 0.00029
002705.79366 ? 0.00005
003124.33167 ? 0.00241
003635.10909 ? 0.00004
004017.54073 ? 0.00012
004030.65493 ? 0.00049
004530.50924 ? 0.00042
004922.90073 ? 0.00037
005109.50184 ? 0.00008
005802.23031 ? 0.00012
005815.64184 ? 0.00055
005952.20898 ? 0.00013
010009.38767 ? 0.00376
010946.34439 ? 0.00013
011057.55198 ? 0.00634
011901.27437 ? 0.00007
012031.66333 ? 0.00003
012421.45944 ? 0.00029
013432.03008 ? 0.00008
013435.66684 ? 0.00014
014043.07794 ? 0.00051
014310.13154 ? 0.00006
014454.09346 ? 0.00034
015123.48911 ? 0.00007
015310.12167 ? 0.00009
015537.05928 ? 0.00020
?322101.2343 ? 0.0014
+605122.8034 ? 0.0036
+201942.3170 ? 0.0005
?344549.6414 ? 0.0085
?295550.0978 ? 0.0055
+114400.7741 ? 0.0013
+062821.2394 ? 0.0007
+204749.7886 ? 0.0018
?305415.2772 ? 0.0417
?302747.4157 ? 0.0044
?415310.7799 ? 0.0124
?395426.0553 ? 0.0013
+611743.5417 ? 0.0008
?303119.3500 ? 0.0013
+243926.2299 ? 0.0006
?420203.9379 ? 0.0097
?260212.6168 ? 0.0019
?351248.7796 ? 0.0082
+092957.7632 ? 0.0011
?392249.3865 ? 0.0524
+143403.6198 ? 0.0010
?324327.8246 ? 0.0024
?322520.3364 ? 0.0168
?390002.9051 ? 0.0104
?311627.3203 ? 0.0142
?422633.2919 ? 0.0014
?323420.7470 ? 0.0036
?334757.4885 ? 0.0204
+581223.6836 ? 0.0005
?333732.0234 ? 0.0984
+613330.4557 ? 0.0003
+563216.9830 ? 0.1780
+082954.6926 ? 0.0019
?270124.6523 ? 0.0009
?341621.4461 ? 0.0108
?384333.3824 ? 0.0016
?093102.8734 ? 0.0031
+634606.8918 ? 0.0007
?320056.6511 ? 0.0013
?393810.5064 ? 0.0085
?343513.8768 ? 0.0015
?331025.8571 ? 0.0030
?404842.3488 ? 0.0075
40
11
68
19
16
40
72
31
4
17
14
43
18
47
69
17
28
49
28
4
46
19
9
15
10
36
20
5
39
3
60
2
15
48
17
28
16
16
31
17
35
24
15
X/S
X/S
X/S
X-o
X/S
X/S
X/S
X-o
X-o
X/S
X/S
X/S
X/S
X/S
X/S
X/S
X/S
X-o
X/S
X/S
X/S
X/S
X/S
X-o
X/S
X/S
X/S
X-o
X/S
X-o
X/S
X/S
X/S
X/S
X/S
X/S
X+
X/S
X/S
X+
X/S
X/S
X/S
0.43
0.12
0.19
0.09
0.10
0.12
0.15
<0.05
0.09
0.11
0.49
1.15
0.09
0.24
0.15
1.13
0.54
0.10
0.10
0.07
0.08
0.09
0.12
0.13
0.12
0.81
0.11
0.11
0.12
0.09
0.26
<0.05
0.28
0.43
0.14
0.31
0.10
0.22
0.15
0.12
0.16
0.35
0.51
3
4
3
4
5
4
2
5
5
4
5
1
4
3
3
1
4
5
3
5
4
4
4
5
4
2
4
5
3
4
2
5
?
4
4
4
5
4
3
4
3
4
4
0.12
0.07
0.24
0.07
<0.05
0.09
0.23
<0.05
0.07
0.16
0.09
0.94
0.26
0.16
0.11
0.31
0.14
0.20
0.12
<0.05
0.15
0.13
0.08
0.06
0.07
0.39
0.05
0.05
0.12
0.05
0.47
<0.05
0.13
0.19
0.08
0.19
0.11
0.19
0.20
0.11
0.11
0.23
0.18
3
4
3
4
5
4
2
5
5
3
5
1
2
3
3
4
4
3
3
5
3
3
4
5
4
2
4
4
3
4
2
5
?
4
4
3
4
4
3
4
4
4
4
2See http://www.nrao.edu/vlba/VCS2.
3See http://magnolia.nrao.edu/vlba_calib.
2564 FOMALONT ET AL. Vol. 126

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J0251+5616 and J0450+4056 have quality factors of 2
and 3, respectively, and are suitable for calibrators, even
at the longest earth baselines. Source J0625+1440, with
quality factor 4, has significant large-scale structure, and
any small-diameter radio component is weak. This cali-
brator should be used with caution and, perhaps, checked
before observations if possible. The source J1755+1820 is
very resolved and is not a useful calibrator. The image is
dominated by noise, and the position is derived only
from the very shortest spacings where some signal was
detected.
4. CONCLUSION
TheVCS2surveyhasadded276newsourcesintheVLBA
calibrator catalog. These additional sources are prefer-
entially near the Galactic plane and in the range ?30?>
? > ?45?, where the density of calibrators was relatively
Fig. 1.—Samples of high-to-low quality calibrators. Each line from left to right shows: 2.3 GHz visibility amplitude vs. projected spacing; 2.3 GHz image;
8.4 GHz visibility amplitude vs. projected spacing; and 8.4 GHz image. The four lines show these plots for four sources in decreasing order of calibration
quality. The visibility plot ordinates are in millijanskys, the abscissa in millions of wavelengths. The image contour plots are labeled with the right ascension
and declination (epoch J2000.0), the gray-scale levels (in millijanskys) are given at the top of each image plot, the peak flux density (in janskys) and contour
levelsare givenatthebottomof eachimageplot.
No. 5, 2003VLBA CALIBRATOR SURVEY2565

Page 5

low. It is beneficial to have a high density of calibrators in
the sky since the closer a calibrator is to the target source,
the smaller its residual phase error will be. This will improve
the quality of the image and the accuracy of the relative
position between the calibrator and the target. The use of
multiple calibrator sources for a target also demands a high
density of calibrator sources.
The authors thank the many staff members of the
Goddard Space Flight Center and the National Radio
Astronomy Observatory for their help and effort. The
National Radio Astronomy Observatory is a facility of the
National Science Foundation operated under cooperative
agreement by Associated Universities, Inc.
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2566 FOMALONT ET AL.