Blazars define a stable celestial reference frame

¹ Celestial Reference Frame Department, U.S. Naval Observatory, 3450 Massachusetts Ave NW, Washington, DC 20392, USA
² LTE, Observatoire de Paris, Université PSL, CNRS UMR8255, Sorbonne Université, Université de Lille, LNE, 61 avenue de l’Observatoire, 75014 Paris, France

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 31 July 2025
Accepted: 9 December 2025

Abstract

Context. Recent work has shown that optical-radio position offsets and radio position variability are inversely correlated with the optical photometric variability of active galactic nuclei (AGN). A key prediction of these findings is that a reference frame constructed using highly photometrically variable AGN should be more stable than a frame that does not account for variability and that photometric variability can be used to optimally weight all sources in order to maximize frame stability.

Aims. To test this prediction by determining the stability of reference frames constructed using sources selected on photometric variability.

Methods. Using ICRF3 matched to Gaia DR3, we employed a bootstrap method to estimate the multi-epoch stability of frames constructed using AGN selected at varying levels of photometric variability. We fit vector spherical harmonics to the coordinate differences between the three ICRF3 frames (S/X, K, and X/Ka) and Gaia DR3 and quantified the statistical dispersion as a function of blazar-like (high variability), quasar-like (low variability), and intermediate-variability class.

Results. An S/X reference frame constructed using blazars exceeds the stability of a frame constructed with quasars by a factor of 6 and is twice as stable as the ICRF3 defining sources. At K and X/Ka, a blazar-based frame matches or exceeds the stability of the defining sources by a factor of 1.4 in the case of X/Ka and exceeds the stability of a frame based on quasars by over a factor of 2 in both cases. The smaller improvement at K and X/Ka is likely arises because sources selected at higher frequency are more likely to be blazars. We derived a variability-based astrometric covariance scaling method that results in factor of 2 reduction in frame distortions and instabilities between ICRF3 S/X and Gaia DR3, with a mild improvement for ICRF3 K but no difference for ICRF3 X/Ka, which is dominated by known distortions.

Conclusions. A celestial reference frame defined using blazars is likely to be stable over time. This confirms the prediction that an optimal weighting of the link between the optical and radio celestial reference frames is enabled by accounting for photometric variability.