Towards a multi-waveband optical-radio ICRF

For two decades, VLBI has been the sole technique able to measure the astrometric positions of extragalactic sources with high accuracy, and hence to realize the International Celestial Reference Frame (ICRF). This led to three successive such realizations, ICRF1 in 1997, ICRF2 in 2009, and ICRF3 in 2018, each of which supplanting the previous realization after adoption by the IAU. The launch of the Gaia space mission 10 years ago changed the situation since it gave the chance to build a highly-accurate extragalactic frame in the optical band for the first time. The Gaia-CRF2 frame was released in 2018, followed two years later by Gaia-CRF3, which was adopted by the IAU as the optical counterpart of the ICRF3 in 2021. This new situation creates the potential for a multi-waveband (optical-radio) ICRF, the study of which was assigned to the IAU working group “Multi-waveband ICRF”, established in 2021. The mandate of the working group is to work toward the realization of such a multi-waveband frame, incorporating positions in both radio and optical bands and ensuring their consistency over the various bands. The final objective is to produce the next generation VLBI frames at the S/X, K, and X/Ka bands, or at any other radio band that may emerge in the coming years, to match these with the optical realization from the Gaia space mission, and to place all such positions on a common grid guaranteeing consistency of the source positions over the different bands. Before this can be accomplished, a number of questions relating to the construction of such a multi-waveband frame are to be addressed. These include dealing with non-uniform sky distributions, agreeing on a consistent treatment of Galactocentric acceleration, establishing common practices to align reference frames in different bands, and considerations regarding wavelength and time-dependent source positions. A proper terminology for referring to the individual (per band) components of the reference frame must also be defined. The targeted date for completion of this future multi-waveband ICRF is 2027, meaning a possible adoption by the IAU General Assembly that will take place the same year.