The Astro2Geo Project I. Radio astrometric offsets correlated with Gamma-ray brightness

TL;DR

This study explores the correlation between radio astrometric offsets of AGN and their Gamma-ray brightness, revealing complex relationships and potential physical mechanisms behind positional shifts.

Contribution

It provides the first large-scale analysis linking radio astrometric variability of AGN to Gamma-ray emission, highlighting the complexity of underlying astrophysical processes.

Findings

Approximately 90% of sources show significant correlations between radio offsets and Gamma-ray flux.

Both positive and negative correlations are observed, with sign differences across frequency bands.

No single physical explanation fully accounts for the observed correlations, indicating multiple mechanisms at play.

Abstract

Precision geodesy relies on the stability of the International Celestial Reference Frame (ICRF), yet its reference sources, Active Galactic Nuclei (AGN), exhibit changes in source structure that can manifest as apparent shifts in their astrometric positions. The high-precision radio measurements used to maintain the ICRF therefore provide a means to investigate the astrophysical mechanisms driving these changes. In particular, the observed astrometric variability offers a unique opportunity to link positional shifts in AGN to high-energy astrophysical processes. We investigated the relationship between the astrometric positions of ICRF AGN and their Gamma-ray emission. We measured the positional offsets of radio cores relative to the ICRF3 at both S/X and K bands and compared them to Fermi-LAT Gamma-ray fluxes within +/-30 days. Out of 92 radio sources, we identified 57 that had enough overlapping data. We find a high incidence of statistically significant (p<0.05) power-law correlations, with ~90% of sources exhibiting this behaviour. The nature of this correlation is complex: we observe both positive and negative correlations, and the sign of the correlation can differ between the two frequency bands for the same source. To explain the correlations, we tested variable Gamma-ray emission locations, changes in nuclear opacity, and variations in jet position angle. Our analysis reveals no single explanation and suggests a complex interplay of multiple physical mechanisms. A search for time lags between the radio position offsets and Gamma-ray fluxes revealed tentative - and highly caveated - evidence for a time-delay in only five sources. A statistical comparison with the OCARS catalogue shows that, although our sample is biased towards optically brighter sources with better-constrained astrometric solutions, it remains representative of the broader AGN population in terms of redshift.