Unprecedented change in the position of four radio sources

TL;DR

This paper reports unprecedented large shifts in the apparent positions of four radio-loud active galactic nuclei over two decades, caused by changes in their radio structures, highlighting the dynamic nature of these sources.

Contribution

It presents new VLBI observations revealing extreme positional changes in four AGNs, emphasizing the impact of radio structure evolution on astrometric measurements.

Findings

Four AGNs showed 20-130 milliarcsec positional shifts.

Shifts are caused by changes in radio source structure.

Long-term monitoring reveals dynamic AGN behavior.

Abstract

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.