Connecting VLBI and Gaia Celestial Reference Frames

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: Introductionmentioning

confidence: 99%

Possible systematics in the VLBI catalogs as seen from Gaia

Zhu

2017

A&A

“…In addition, the celestial reference frame, based on the subset of extragalactic sources, has some systematic effects, such as global rotation and deformation, which results from the apparent proper motions. Recently Malkin (2016) took the Galactic aberration effect into consideration and provided possible methods to create a future link of the Gaia reference frame with the next generation of ICRF.…”

Section: Overall Property Of the Auxiliary Quasar Solutionmentioning

confidence: 99%

Overall properties of theGaiaDR1 reference frame

Zhu

et al. 2017

A&A

Aims. The first Gaia data release (Gaia DR1) provides 2 191 ICRF2 sources with their positions in the auxiliary quasar solution and five astrometric parameters -positions, parallaxes, and proper motions -for stars in common between the Tycho-2 catalogue and Gaia in the joint Tycho-Gaia astrometric solution (TGAS). We aim to analyze the overall properties of Gaia DR1 reference frame. Methods. We compare quasar positions of the auxiliary quasar solution with ICRF2 sources using different samples and evaluate the influence on the Gaia DR1 reference frame owing to the Galactic aberration effect over the J2000.0-J20015.0 period. Then we estimate the global rotation between TGAS with Tycho-2 proper motion systems to investigate the property of the Gaia DR1 reference frame. Finally, the Galactic kinematics analysis using the K-M giant proper motions is performed to understand the property of Gaia DR1 reference frame.Results. The positional comparison between the auxiliary quasar solution and ICRF2 shows negligible orientation and validates the declination bias of ∼−0.1 mas in Gaia quasar positions with respect to ICRF2. Galactic aberration effect is thought to cause an offset ∼0.01 mas of the Z axis direction of Gaia DR1 reference frame. The global rotation between TGAS and Tycho-2 proper motion systems, obtained by different samples, shows a much smaller value than the claimed value 0.24 mas yr −1 . For the Galactic kinematics analysis of the TGAS K-M giants, we find possible non-zero Galactic rotation components beyond the classical Oort constants: the rigid part ω Y G = −0.38±0.15 mas yr −1 and the differential part ω ′ Y G = −0.29±0.19 mas yr −1 around the Y G axis of Galactic coordinates, which indicates possible residual rotation in Gaia DR1 reference frame or problems in the current Galactic kinematical model. Conclusions. The Gaia DR1 reference frame is well aligned to ICRF2, and the possible influence of the Galactic aberration effect should be taken into consideration for the future Gaia-ICRF link. The cause of the rather small global rotation between TGAS and Tycho-2 proper motion systems is unclear and needs further investigation. The possible residual rotation in Gaia DR1 reference frame inferred from the Galactic kinematic analysis should be noted to and examined in future data release.

“…These two quantities are generally closely correlated, but this correlation is less strong for sources with poor history of observations. As was found in Malkin (2016), the dependence of the source position uncertainty on observation statistics is better described by the number of observations than by the number of sessions. To check the suggestion that the outliers are mostly observed for sources having a few observations, a corresponding test was performed and its results are presented in Figure 3.…”

Section: Icrf Catalogsmentioning

confidence: 68%

How Well is the International Celestial Reference System Maintained in Official IAU Implementations?

Malkin

2024

Self Cite

The International Celestial Reference Frame (ICRF) based on the VLBI-derived positions of 608 extragalactic radio sources was adopted by the International Astronomical Union (IAU) in 1998 as the first realization of the International Celestial Reference System (ICRS). Later, in 2009 and 2020, two extended ICRF versions, ICRF2 and ICRF3, respectively, were released. The latter is adopted by the IAU as the current implementation of the ICRF in the radio band. In the meantime, the Gaia mission delivered three versions of the optical ICRS realization in 2016, 2018, and 2022 with an accuracy similar to that achieved by VLBI-based ICRF. The Gaia-CRF catalogs were linked to the ICRF under no-rotation conditions and thus may suffer from ICRF systematic instability if the latter is substantial. In this work, a new analysis was performed to assess the long-term stability of radio and optical ICRS realizations. Based on the 16-parameter vector spherical harmonics expansion of the differences between the three ICRF catalogs, it can be concluded that the mutual orientation between them is at a level of a few tens of microarcseconds, while the components of the glide vector and E 2,0 term are several times greater. A comparison of the three Gaia-CRF catalogs with the ICRF3-SX showed that for the latest Gaia-CRF catalog, Gaia-CRF3, all rotational and deformation components are below 20 μas except for the E 2,0 term, which is several times greater. For both ICRF and Gaia-CRF catalogs, the evolution of the source position errors is also tracked.