J/A+A/644/A159 The third realization of the ICRF, ICRF3 (Charlot+, 2020)
The third realization of the International Celestial Reference Frame by
very long baseline interferometry.
Charlot P., Jacobs C.S., Gordon D., Lambert S., de Witt A., Boehm J.,
Fey A.L., Heinkelmann R., Skurikhina E., Titov O., Arias E.F., Bolotin S.,
Bourda G., Ma C., Malkin Z., Nothnagel A., Mayer D., MacMillan D.S.,
Nilsson T., Gaume R.
<Astron. Astrophys. 644, A159 (2020)>
=2020A&A...644A.159C 2020A&A...644A.159C (SIMBAD/NED BibCode)
ADC_Keywords: VLBI ; Radio sources ; Positional data ; Active gal. nuclei ; QSOs
Keywords: reference systems - astrometry - techniques: interferometric -
quasars: general - galaxies: nuclei - radio continuum: general
Abstract:
A new realization of the International Celestial Reference Frame
(ICRF) is presented based on the work achieved by a working group of
the International Astronomical Union (IAU) mandated for this purpose.
This new realization follows the initial realization of theICRF
completed in 1997 and its successor, ICRF2, adopted as a replacement
in 2009. The new frame, referred to as ICRF3, is based on nearly 40
years of data acquired by very long baseline interferometry at the
standard geodetic and astrometric radio frequencies (8.4 and 2.3GHz),
supplemented with data collected at higher radio frequencies (24GHz
and dual-frequency 32 and 8.4GHz) over the past 15 years.
State-of-the-art astronomical and geophysical modeling has been used
to analyze these data and derive source positions. The modeling
integrates, for the first time, the effect of the galactocentric
acceleration of the solar system (directly estimated from the data)
which, if not considered, induces significant deformation of the frame
due to the data span. The new frame includes positions at 8.4GHz for
4536 extragalactic sources. Of these, 303 sources, uniformly
distributed on the sky, are identified as "defining sources" and as
such serve to define the axes of the frame. Positions at 8.4GHz are
supplemented with positions at 24GHz for 824 sources and at 32GHz for
678 sources. In all, ICRF3 comprises 4588 sources, with
three-frequency positions available for 600 of these. Source positions
have been determined independently at each of the frequencies in order
to preserve the underlying astrophysical content behind such positions.
They are reported for epoch 2015.0 and must be propagated for
observations at other epochs for the most accurate needs, accounting
for the acceleration toward the Galactic center, which results in a
dipolar proper motion field of amplitude 0.0058 milliarcsecond/yr
(mas/yr). The frame is aligned onto the International Celestial
Reference System to within the accuracy of ICRF2 and shows a median
positional uncertainty of about 0.1mas in right ascension and 0.2 mas
in declination, with a noise floor of 0.03mas in the individual source
coordinates. A subset of 500 sources is found to have extremely
accurate positions, in the range of 0.03 to 0.06mas, at the
traditional 8.4GHz frequency. Comparing ICRF3 with the recently
released Gaia Celestial Reference Frame 2 in the optical domain, there
is no evidence for deformations larger than 0.03 mas between the two
frames, in agreement with the ICRF3 noise level. Significant positional
offsets between the three ICRF3 frequencies are detected for about 5%
of the sources.Moreover, a notable fraction (22%) of the sources shows
optical and radio positions that are significantly offset. There are
indications that these positional offsets may be the manifestation of
extended source structures. This third realization of the ICRF was
adopted by the IAU at its 30th General Assembly in August 2018 and
replaced the previous realization, ICRF2, on January 1, 2019.
Description:
Tables 10-12 present the coordinates of the 4588 sources comprised in
ICRF3 along with their uncertainties. Table 10 is for the S/X band
frame and includes 4536 sources. Table 11 is for the K band frame and
includes 824 sources. Table 12 is for the X/Ka band frame and includes
678 sources. Besides source coordinates, these tables also provide
proper information to identity each source (ICRF designation and IERS
name) and details about the VLBI sessions (first and last session in
which a source was observed, mean epoch of the sessions, number of
sessions), the observations (number of VLBI delays and delay rates
used to estimate the source position), and the characteristics of the
errors (correlation coefficient between right ascension and
declination).
Tables 14-16 list the sources for which the normalized separation
between the S/X band position and (i) the K band position, (ii) the
X/Ka band position, and (iii) the Gaia-CRF2 position, is above 3.
Position differences are characterized by the right ascension and
declination offsets and the length and direction of the offset vector
joining the two positions being compared. The normalized separation is
defined as the length of the offset vector divided by its formal
uncertainty.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table10.dat 164 4536 Coordinates at S/X band for 4536 sources
table11.dat 164 824 Coordinates at K band for 824 sources
table12.dat 164 678 Coordinates at X/Ka band for 678 sources
table14.dat 72 46 List of sources with normalized separation
between the S/X and K band positions > 3
table15.dat 72 70 List of sources with normalized separation
between the S/X and X/Ka band positions > 3
table16.dat 72 653 List of sources with normalized separation
between the S/X and Gaia-CRF2 positions > 3
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See also:
J/AJ/127/3587 : VLBI ICRF. II (Fey+, 2004)
I/323 : International Celestial Reference Frame 2, ICRF2 (Ma+, 2009)
Byte-by-byte Description of file: table10.dat table11.dat table12.dat
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Bytes Format Units Label Explanations
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1- 21 A21 --- ICRF International Celestial Reference Frame name,
(JHHMMSS.s+DDMMSS) (1)
26- 33 A8 --- IERS International Earth Rotation and Reference
Systems Service name
(HHMM+DDd, B1950 equinox) (2)
36 A1 --- Cat [D] ICRF3 source category (3)
37 A1 --- fVLBA [V] Flag signaling that the observations
are solely from the VLBA (4)
42- 43 I2 h RAh Right ascension (ICRF) at Ep=2015.0
45- 46 I2 min RAm Right ascension (ICRF) at Ep=2015.0
48- 58 F11.8 s RAs Right ascension (ICRF) at Ep=2015.0
63 A1 --- DE- Declination sign (ICRF) at Ep=2015.0
64- 65 I2 deg DEd Declination (ICRF) at Ep=2015.0
67- 68 I2 arcmin DEm Declination (ICRF) at Ep=2015.0
70- 79 F10.7 arcsec DEs Declination (ICRF) at Ep=2015.0
85- 94 F10.8 s e_RAs Right ascension uncertainty
100-108 F9.7 arcsec e_DEs Declination uncertainty
110-116 F7.4 --- Corr Correlation coefficient between
right ascension and declination
120-126 F7.1 d Ep Mean epoch of the sessions in which the
source was observed (5)
129-135 F7.1 d Of Epoch of first session in which the source
was observed (5)
138-144 F7.1 d Ol Epoch of last session in which the source
was observed (5)
146-150 I5 --- Nses Number of sessions in which the source
was observed
152-157 I6 --- Ndel Number of delay observations used to
estimate the source position
159-164 I6 --- Nrat Number of delay rate observations used to
estimate the source position
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Note (1): The ICRF designations were derived from the source coordinates with
the format ICRF JHHMMSS.s+DDMMSS or ICRF JHHMMSS.s-DDMMSS, according to the
original ICRF prescriptions.
Note (2): The complete format for the IERS names, constructed from previous
B1950.0 coordinates, includes acronym and epoch and is IERS BHHMM+DDd
or IERS BHHMM-DDd.
Note (3): ICRF3 defining sources are identified with a "D".
Note (4): Sources observed solely with the VLBA are identified with a "V"
(not in table12.dat).
Note (5): Dates are given as Modified Julian Date (MJD), i.e. JD-2400000.5.
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Byte-by-byte Description of file: table14.dat table15.dat table16.dat
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Bytes Format Units Label Explanations
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1- 8 A8 --- IERS IERS name
10- 16 I7 uas oRAcosDE Offset in RAcosDE (right ascension scaled
by the cosine of the declination)
18- 22 I5 uas e_oRAcosDE Uncertainty for the RAcosDE offset
24- 30 I7 uas oDE Offset in declination
32- 36 I5 uas e_oDE Uncertainty for the declination offset
42- 47 I6 uas Sep Length of the offset vector (1)
49- 53 I5 uas e_Sep Uncertainty for the offset vector length
57- 60 I4 deg PA Direction of the offset vector (2)
62- 64 I3 deg e_PA Uncertainty for the offset vector direction
68- 72 F5.1 --- Sep/e_Sep Normalized separation
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Note (1): The vector length is also denoted as angular separation in the text.
Note (2): The vector direction is counted counter clockwise from north to east.
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Acknowledgements:
Patrick Charlot, patrick.charlot(at)u-bordeaux.fr
(End) Patricia Vannier [CDS] 29-Sep-2020