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. Using new VLBI observations in 2018-2021, we show here that four extragalactic radio sources (3C48, CTA21, 1144+352, 1328+254) have a dramatic shift in their positions by 20-130 milliarcsec over two decades. For all four sources, the apparent positional shift is caused by their radio structure change.
@article{arxiv.2202.13119,
title = {Unprecedented change in the position of four radio sources},
author = {Oleg Titov and Sándor Frey and Sébastien Lambert and Fengchun Shu and Bo Xia and Javier González and Belén Tercero and Sergey Gulayev and Stuart Weston and Tim Natusch},
journal= {arXiv preprint arXiv:2202.13119},
year = {2022}
}