Synergy between VLBI and Gaia: towards a multi-waveband celestial reference frame

A new IAU working group entitled “Multi-waveband ICRF (International Celestial Reference Frame)” has been established in 2021. The objective of this working group is to work toward the realization of a multi-waveband celestial reference frame, incorporating positions in both radio and optical bands and ensuring their consistency over the various bands. The end goal of the working group will be 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. This future multi-waveband ICRF should be a valuable asset to further improve our understanding of the physics of the underlying extragalactic objects.