Opacity in compact extragalactic radio sources and its effect on astrophysical and astrometric studies

Understand

The apparent position of the "core" in a parsec-scale radio jet (a compact, bright emitting region at the narrow end of the jet) depends on the observing frequency, owing to synchrotron self-absorption and external absorption.

• While providing a tool probing physical conditions in the vicinity of the core, this dependency poses problems for astrometric studies using compact radio sources.
• We investigated the frequency-dependent shift in the positions of the cores (core shift) observed with very long baseline interferometry (VLBI) in parsec-scale jets.
• We discuss related physics, as well as its effect on radio astrometry and the connection between radio and optical positions of astrometric reference objects.