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

TL;DR

This study reveals an inverse relationship between photometric variability and optical-radio position offsets in AGNs, with highly variable sources being predominantly blazars with aligned jets, impacting celestial reference frame accuracy.

Contribution

It is the first to demonstrate the inverse correlation between photometric variability and position offsets in AGNs using Gaia DR3 data, highlighting the role of jet orientation.

Findings

Highly variable AGNs have lower position offsets.

Most variable AGNs are blazars with gamma-ray emission.

Variability can inform optimal weighting of celestial reference frames.

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.