SIMBAD references

Astrometric positions of radio-emitting active galactic nuclei (AGNs) can be determined with sub-milliarcsec accuracy using very long baseline interferometry (VLBI). The usually small apparent proper motion of distant extragalactic targets allow us to realize the fundamental celestial reference frame with VLBI observations. However, long-term astrometric monitoring may reveal extreme changes in some AGN positions. We present the case of CTA 21 whose position gradually shifted by ∼0.″04 over two decades. By comparing archival VLBI images from 1996 and 2018, we show that its radio structure changed substantially, likely causing the apparent positional shift.