The reference frame determined by Gaia EDR3, Gaia-CRF3, is aligned to the International Celestial Reference System by referring to counterparts in its latest realization, the third International Celestial Reference Frame (ICRF3), which is calculated from very long baseline interferometry (VLBI) observations of extragalactic objects at radio frequencies.
The objects in ICRF3, although bright at radio frequencies, are mostly faint at optical frequencies.
The non-rotation of the optically bright Gaia frame to ICRF3 has to be tested separately because the Gaia dataset is known to be magnitude-dependent in terms of astrometric calibration. This can be done by identifying additional counterparts besides objects in ICRF3.
Suitable counterparts are radio stars observed by VLBI relative to extragalactic objects in ICRF3 using phase-referencing.
We discuss the rotational differences, i.e., orientation and spin, between the optically bright Gaia EDR3 and models of stellar motion from VLBI. In particular, we show the effects of improved models of stellar motion, for which we extended the time series from literature or archives with new VLBI results.