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

Abstract Precision geodesy relies on the stability of the International Celestial Reference Frame (ICRF), yet its reference sources, Active Galactic Nuclei (AGN), exhibit intrinsic 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 powerful means to investigate the astrophysical mechanisms driving these position changes. In particular, the observed astrometric variability offers a unique opportunity to link positional shifts in AGN to high-energy astrophysical processes. We therefore investigated the relationship between the astrometric positions of ICRF AGN and their γ -ray emission. We measured the positional offsets of radio cores relative to the third realisation of the ICRF at both S/X (2.3/8.4 GHz) and K (24 GHz) bands and compared them to Fermi- LAT (Large Area Telescope) γ -ray fluxes within ±30 days of the radio observation. Out of an initial sample of 92 radio sources selected for having extensive radio astrometric observations, we identified 57 that met our selection criteria of having sufficient overlapping γ -ray data points to allow for regression analysis. 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 several scenarios, including variable γ -ray emission locations, changes in nuclear opacity, and variations in jet position angle. Our analysis reveals no single, universally applicable explanation. Instead, the results suggest that the observed correlation is driven by a complex interplay of multiple physical mechanisms, the dominance of which likely varies between sources. A search for time lags between the radio position offsets and γ -ray fluxes revealed tentative - and highly caveated - evidence for a time-delay in only five sources, with no evidence in other sources. A statistical comparison with the Optical Characteristics of Astrometric Radio Sources (OCARS) catalogue shows that, although our sample is biased towards optically brighter sources with better-constrained astrometric solutions due to their larger number of radio observations, it remains representative of the broader AGN population in terms of redshift distribution.