Comparison of VLBI radio source catalogs

Zhu

2017

A&A

Aims. In order to investigate the systematic errors in the very long baseline interferometry (VLBI) positions of extragalactic sources (quasars) and the global differences between Gaia and VLBI catalogs, we use the first data release of Gaia (Gaia DR1) quasar positions as the reference and study the positional offsets of the second realization of the International Celestial Reference Frame (ICRF2) and the Goddard VLBI solution 2016a (gsf2016a) catalogs. Methods. We select a sample of 1032 common sources among three catalogs and adopt two methods to represent the systematics: considering the differential orientation (offset) and declination bias; analyzing with the vector spherical harmonics (VSH) functions. Results. Between two VLBI catalogs and Gaia DR1, we find that: i) the estimated orientation is consistent with the alignment accuracy of Gaia DR1 to ICRF, of ~0.1 mas, but the southern and northern hemispheres show opposite orientations; ii) the declination bias in the southern hemisphere between Gaia DR1 and ICRF2 is estimated to be +152 μas, much larger than that between Gaia DR1 and gsf2016a which is +34 μas. Between two VLBI catalogs, we find that: i) the rotation component shows that ICRF2 and gsf2016a are generally consistent within 30 μas; ii) the glide component and quadrupole component report two declination-dependent offsets: dipolar deformation of ~+50 μas along the Z-axis, and quadrupolar deformation of ~−50 μas that would induce a pattern of sin2δ. Conclusions. The significant declination bias between Gaia DR1 and ICRF2 catalogs reported in previous studies is possibly attributed to the systematic errors of ICRF2 in the southern hemisphere. The global differences between ICRF2 and gsf2016a catalogs imply that possible, mainly declination-dependent systematics exit in the VLBI positions and need further investigations in the future Gaia data release and the next generation of ICRF.

Section: Introductionsupporting

confidence: 85%

Section: Introductionmentioning

confidence: 99%

Possible systematics in the VLBI catalogs as seen from Gaia

Zhu

2017

A&A

“…An accurate CRF opens the doors to millimeterlevel geodetic measurements (e.g., Earth rotation), insights into the Earth internal structure (e.g., Mathews et al 2002), and testing of General Relativity and alternative theories with VLBI (Le Poncin-Lafitte et al 2016, and references therein). The arrival of the Gaia Celestial Reference Frame (Gaia-CRF2, Gaia Collaboration et al 2016) -the first time that one can compare two independent CRF realizations with similar high accuracies -also allows comparisons between the positions of the common objects at several frequencies, bringing insights into the physics of quasars (e.g., Kovalev et al 2017), reference frames (e.g., Lambert 2014), and complex structures made of one or several black holes (e.g., Roland et al 2015).…”

Section: Introductionmentioning

confidence: 99%

Systematics and accuracy of VLBI astrometry: A comparison with Gaia Data Release 2

Lambert

Zhu

et al. 2020

A&A

Context. The third generation of the ICRF -ICRF3 -was published in 2018. This new fundamental catalog provides radio source positions measured independently at three bands: S/X, K, and X/Ka, representing three independent radio celestial frames which altogether constitute together a multi-frequency ICRF. Aims. We aim to investigate the overall properties of the ICRF3 with the help of the Gaia Data release 2 (Gaia DR2). This could serve as an external check of the quality of the ICRF3. Methods. The radio source positions of the ICRF3 catalog were compared with the Gaia DR2 positions of their optical counterparts at G < 18.7. Their properties were analyzed in terms of the dependency of the quoted error on the number of observations, on the declination, and the global difference, the latter revealed by means of expansions in the vector spherical harmonics. Results. The ICRF3 S/X-band catalog shows a more smooth dependency on the number of observations than the ICRF1 and ICRF2, while the K and X/Ka-band yield a dependency discrepancy at the number of observations of ∼50. The rotation of all ICRF catalogs show consistent results, except for the X-component of the X/Ka-band which arises from the positional error in the non-defining sources. No significant glides were found between the ICRF3 S/X-band component and Gaia DR2. However, the K-and X/Kaband frames show a dipolar deformation in Y-component of +50 µas and several quadrupolar terms of 50 µas in an absolute sense. A significant glide along Z-axis exceeding 200 µas in the X/Ka-band was also reported. These systematics in the ICRF catalog are shown to be less dependent on the limiting magnitude of the Gaia sample when the number of common sources is sufficient (> 100). Conclusions. The ICRF3 S/X-band catalog shows improved accuracy and systematics at the level of noise floor. But the zonal errors in the X/Ka-band should be noted, especially in the context of comparisons of multi-frequency positions for individual sources.

“…2). The parameter D 3 is directly connected to the source declination, as it describes a glide toward the z-axis and thus is susceptible to many modeling and analysis choices during the processing of the VLBI data and linked to the so-called declination bias (e.g., Lambert 2014, and references therein). This term describes an artificial displacement of the sources toward the poles and is mainly associated with the asymmetry of the VLBI network.…”

Section: Extended Quality Assessment Of Input To Combinationmentioning

confidence: 99%

Realization of a multifrequency celestial reference frame through a combination of normal equation systems

Karbon

Nothnagel

2019

A&A

Context. We present a celestial reference frame (CRF) based on the combination of independent, multifrequency radio source position catalogs using nearly 40 years of Very Long Baseline Interferometry observations at the standard geodetic frequencies at SX band and about 15 years of observations at higher frequencies (K and XKa). The final catalog contains 4617 sources. Aims. We produce a multifrequency catalog of radio source positions with full variance-covariance information across all radio source positions of all input catalogs. Methods. We combined three catalogs, one observed at 8 GHz (X band), one at 24 GHz (K band) and one at 32 GHz (Ka band). Rather than only using the radio source positions, we developed a new, rigorous combination approach by carrying over the full covariance information through the process of adding normal equation systems. Special validation routines were used to characterize the random and systematic errors between the input reference frames and the combined catalog.Results. The resulting CRF contains precise positions of 4617 compact radio astronomical objects, 4536 measured at 8 Ghz, 824 sources also observed at 24 GHz, and 674 at 32 GHz. The frame is aligned with ICRF3 within ±3 µas and shows an average positional uncertainty of 0.1 mas in right ascension and declination. No significant deformations can be identified. Comparisons with Gaia-CRF remain inconclusive, nonetheless significant differences between all frames can be attested.