摘要：Aims.In order to investigate the systematic errors in the very long baseline interferometry (VLBI) positions of extragalactic sources (quasars) and the global differences betweenGaiaand VLBI catalogs, we use the first data release ofGaia(GaiaDR1) 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 andGaiaDR1, we find that: i) the estimated orientation is consistent with the alignment accuracy ofGaiaDR1 to ICRF, of~0.1 mas, but the southern and northern hemispheres show opposite orientations; ii) the declination bias in the southern hemisphere betweenGaiaDR1 and ICRF2 is estimated to be+152μas, much larger than that betweenGaiaDR1 and gsf2016a which is+34μas. Between two VLBI catalogs, we find that: i) the rotation component shows that ICRF2 and gsf2016a are generally consistent within30μas; ii) the glide component and quadrupole component report two declination-dependent offsets: dipolar deformation of~+50μasalong theZ-axis, and quadrupolar deformation of~−50μasthat would induce a pattern ofsin2δ.Conclusions.The significant declination bias betweenGaiaDR1 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 futureGaiadata release and the next generation of ICRF.
