J/AJ/169/173 Observations & positions of 377 AGNs using VLBI (Xu+, 2025)
Variations of absolute source positions determined from quad-band VLBI
observations.
Xu M.H., Charlot P.
<Astron. J. 169, 173 (2025)>
=2025AJ....169..173X 2025AJ....169..173X (SIMBAD/NED BibCode)
ADC_Keywords: Active gal. nuclei; VLBI; Positional data; Galaxies, radio;
Interferometry; References; QSOs
Keywords: Active galactic nuclei; Equatorial coordinate system
Abstract:
Active galactic nuclei (AGNs) observed with the very long baseline
interferometry (VLBI) technique are used as fiducial references on the
sky to precisely measure the shape and orientation of the Earth. Their
positions form a celestial reference frame that plays an important
role in both astronomy and geodesy. This study investigates the
accuracy and stability of the positions of the AGNs that are measured
by simultaneous VLBI observations at 3.3, 5.5, 6.6, and 10.5GHz. Based
on position time series from dedicated geodetic solutions, we
characterize the observed source position variations and identify the
possible factors causing such variations. We find that the primary
contributor is source structure for sources above 20° decl., while
the sensitivity of the observations to the decl. coordinate
predominates for sources below 20° decl. The position time series
are further explored to derive more realistic uncertainties for the
quad-band positions. Significant position offsets with respect to the
positions at 2.2/8.6GHz are found for 15% of the sources. For 6% of
the sources, the offsets are larger than 0.8mas. The source structure
may be divided into two parts: the invisible structure (within the
beam size) and the visible structure (on larger scales). The latter
causes closure delays enlarging postfit delay residuals in geodetic
solutions, whereas the former causes source position changes. Such
position changes will contribute significantly to the offsets between
radio and optical positions. Overall, this work highlights the
necessity to have a specific quad-band catalog for processing
operational quad-band observations.
Description:
The quad-band VLBI data used in this study are the 24hr experiments
that are coordinated by the International VLBI Service for Geodesy and
Astrometry (IVS). These data are publicly available from the IVS data
archive. In a typical quad-band VLBI experiment (session), group delay
observations are produced by a network of about 10 antennas for tens
to hundreds of radio sources during a period of 24hr. Overall, each
session includes an average of about 9000 observations.
We analyzed 177 quad-band experiments conducted with a total of
13 antennas from 2017 December 3 to 2024 June 26 using NASA VLBI data
analysis software pSolve--originated from Calc/Solve software. The
theoretical delay model applied follows the IERS 2010 conventions with
the Galactic aberration effect (using the aberration constant of
5.8uas/yr) taken into account.
Forty-six radio sources served for the datum definition of the
celestial reference frame (CRF), i.e., the no-net-rotation constraints
were applied with respect to their ICRF3 positions. In the solution,
the positions of seven sources were estimated per session as local
parameters. The selection of the 46 datum sources--27 of them are
ICRF3 defining sources--and the seven local sources is based on the
analysis of their position stability. Fifty-seven radio sources that
have fewer than 20 group delays accumulated over the 177 sessions and
seven radio sources that have fewer than 30 accumulated group delays
and no more than six group delays in any single session were excluded.
In total, 1,633,781 group delays on 377 radio sources were included in
the global solutions.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 133 377 Source position catalog from quad-band VLBI
observations
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See also:
I/151 : Precise Radio Source Positions from Mark III VLBI (Ma+ 1990)
I/251 : VLBI International Celestial Reference Frame (ICRF) (Ma+, 1997)
I/323 : International Celestial Reference Frame 2, ICRF2 (Ma+, 2009)
I/325 : Radio-optical reference frame link (Zacharias+ 2013)
J/AJ/127/3587 : VLBI ICRF. II (Fey+, 2004)
J/AZh/81/1032 : VLBI Position Measurements (Titov, 2004)
J/AJ/127/3609 : VLBI observations of southern ICRF sources (Ojha+, 2004)
J/AJ/130/2529 : 8.4GHz VLBI observations of Southern ICRF sources (Ojha+, 2005)
J/A+A/493/317 : Radio source selection for the ICRF (Lambert+, 2009)
J/AJ/139/1695 : Celestial Reference Frame at 24 and 43 GHz (Lanyi+, 2010)
J/AJ/139/1713 : Celestial Reference Frame at 24 and 43GHz. II (Charlot+, 2010)
J/AJ/142/35 : The VLBA Galactic Plane Survey (VGaPS) (Petrov+, 2011)
J/AJ/142/89 : Precise astrometry with VLBA (VIPS) (Petrov+, 2011)
J/MNRAS/430/2633 : The epoch ICRF (Xu+, 2013)
J/A+A/553/A122 : Time stability of the ICRF2 axes (Lambert, 2013)
J/MNRAS/445/845 : Implications of the Galac. aberration for CRF (Malkin, 2014)
J/AJ/150/58 : VLBI ICRF2 (Fey+, 2015)
J/A+A/578/A67 : AGN in IFRS. VLBA observations (Herzog+, 2015)
J/AJ/151/154 : Second epoch VLBA Calibrator Survey (VCS-II) (Gordon+, 2016)
J/MNRAS/467/L71 : Significance of VLBI/Gaia position offsets (Petrov+, 2017)
J/ApJS/234/12 : MOJAVE XV.VLBA 15GHz obs. of AGN jets 1996-2016 (Lister+, 2018)
J/A+A/618/A80 : Astrometric classification of 647 VLBI sources (Gattano+, 2018)
J/ApJS/242/5 : 40yr of geodetic/astrometric VLBI obs. of ICRF srcs (Xu+, 2019)
J/MNRAS/493/L54 : Optical polarization properties of AGNs (Kovalev+, 2020)
J/A+A/644/A159 : The third realization of the ICRF, ICRF3 (Charlot+, 2020)
J/A+A/651/A64 : Position difference vectors and of the jets (Lambert+, 2021)
J/AJ/165/139 : The celestial ref. frame at K-band; imaging.I (de Witt+, 2023)
J/ApJS/274/28 : Metrics of astrometric variability in the ICRF.I (Cigan+, 2024)
J/A+A/684/A93 : Position and photometric variability (Lambert+, 2024)
J/A+A/691/A35 : Optical polarimetry of 152 AGN (Blinov+, 2024)
J/ApJS/276/38 : The Radio Fundamental Catalog - RFC (Petrov+, 2025)
http://www.iers.org/IERS/EN/Publications/TechnicalNotes/tn36 : IERS Convention
website (2010)
http://cddis.nasa.gov/archive/vlbi/ivsdata/vgosdb : IVS archive (2012)
Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 21 A21 --- ICRF International Celestial Reference Frame
identifier (JHHMMSS.s+DDMMSS)
23- 30 A8 --- IERS International Earth Rotation and Reference
Systems Service identifier
(HHMM+DDd, B1950 equinox)
32 A1 --- Cat Source category (1)
34- 35 I2 h RAh Hour of Right Ascension (ICRF) at Ep=2015.0
37- 38 I2 min RAm Minute of Right Ascension (ICRF) at Ep=2015.0
40- 50 F11.8 s RAs Second of Right Ascension (ICRF) at Ep=2015.0
52 A1 --- DE- Sign of the Declination (ICRF) at Ep=2015.0
53- 54 I2 deg DEd Degree of Declination (ICRF) at Ep=2015.0
56- 57 I2 arcmin DEm Arcminute of Declination (ICRF) at Ep=2015.0
59- 68 F10.7 arcsec DEs Arcsecond of Declination (ICRF) at Ep=2015.0
70- 79 F10.8 s e_RAs [5.5e-7/5e-5] Uncertainty in RAs
81- 89 F9.7 arcsec e_DEs [4.9e-6/7.7e-4] Uncertainty in DEs
91- 97 F7.4 --- Correl [-0.9/0.4] Correlation between RA and DE
coordinates
99- 105 F7.1 d [58861/60403] Mean Modified Julian Date
(JD-2400000.5) of sessions where the source
was observed
107- 113 F7.1 d 1stMJD [58090/60403] Modified Julian Date
(JD-2400000.5) of first session
115- 121 F7.1 d LastMJD [58910/60487] Modified Julian Date
(JD-2400000.5) of last session
123- 125 I3 --- Nses [1/174] Number of sessions
127- 131 I5 --- Ndel [20/96951] Number of group delay observations
used to estimate its coordinates
133 I1 --- Nrat [0/0] Number of delay rate observations used
(2)
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Note (1): Source category as follows:
C = a datum source (46 occurrences)
S = a source with a mean position from the session-wise position estimates
(7 occurrences)
Note (2): This is kept only for consistency with the ICRF catalogs.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Robin Leichtnam [CDS] 05-Jan-2026