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.

可以使用超长基线干涉测量法 (VLBI) 以亚毫弧秒的精度确定放射性活动星系核 (AGN) 的天体测量位置。遥远的河外目标通常很小的表观自行使我们能够通过 VLBI 观测实现基本天体参考系。然而，长期的天体测量监测可能会揭示一些 AGN 位置的极端变化。我们介绍了 CTA 21 的情况，其位置在 20 年内逐渐移动了 ∼0.”04。通过比较 1996 年和 2018 年的档案 VLBI 图像，我们发现其无线电结构发生了巨大变化，可能导致明显的位置偏移。