Optical-Radio Position Offsets are Inversely Correlated with AGN Photometric Variability

Abstract

Using photometric variability information from the new Gaia DR3 release, I show for the first time that photometric variability is inversely correlated with the prevalence of optical-radio position offsets in the active galactic nuclei (AGNs) that comprise the International Celestial Reference Frame (ICRF). While the overall prevalence of statistically significant optical-radio position offsets is $11\%$, objects with the largest fractional variabilities exhibit an offset prevalence of only $\sim2\%$. These highly variable objects have redder optical color and steeper optical spectral indices indicative of blazars, in which the optical and radio emission is dominated by a line-of-sight jet, and indeed nearly $\sim100\%$ of the most variable objects have $\gamma$-ray emission detected by Fermi LAT. This result is consistent with selection on variability preferentially picking jets pointed closest to the line-of-sight, where the projected optical-radio position offsets are minimized and jet emission is maximally boosted in the observed frame. While only $\sim9\%$ of ICRF objects exhibit such large photometric variability, these results suggest that taking source variability into account may provide a means of optimally weighting the optical-radio celestial reference frame link.