# Observational consequences of optical band milliarcsecond-scale structure in active galactic nuclei discovered by Gaia

## TL;DR

This paper explores how optical structures in active galactic nuclei, observed through Gaia and VLBI, influence positional measurements, revealing insights into optical jets and their variability at milliarcsecond scales.

## Contribution

It demonstrates the impact of optical jet structures on Gaia and VLBI position offsets and proposes using their correlation with optical variability to study jet regions and flare locations.

## Key findings

- Gaia position shifts are influenced by optical jet structures.
- Optical jet variability can cause milliarcsecond-scale position jitter.
- Correlation between VLBI/Gaia offsets and optical light curves can locate flare regions.

## Abstract

We interpret the recent discovery of a preferable VLBI/Gaia offset direction for radio-loud active galactic nuclei (AGNs) along the parsec-scale radio jets as a manifestation of their optical structure on scales of 1 to 100 milliarcseconds. The extended jet structure affects the Gaia position stronger than the VLBI position due to the difference in observing techniques. Gaia detects total power while VLBI measures the correlated quantity, visibility, and therefore, sensitive to compact structures. The synergy of VLBI that is sensitive to the position of the most compact source component, usually associated with the opaque radio core, and Gaia that is sensitive to the centroid of optical emission, opens a window of opportunity to study optical jets at milliarcsecond resolution, two orders of magnitude finer than the resolution of most existing optical instruments. We demonstrate that strong variability of optical jets is able to cause a jitter comparable to the VLBI/Gaia offsets at a quiet state, i.e. several milliarcseconds. We show that the VLBI/Gaia position jitter correlation with the AGN optical light curve may help to locate the region where the flare occurred, estimate its distance from the super-massive black hole and the ratio of the flux density in the flaring region to the total flux density.