Gaia Sees Blazars Move: Locating Optical Flares Using Astrometry

TL;DR

This paper demonstrates that Gaia astrometry can be used to geometrically localize optical flares in blazars to within less than 1 milliarcsecond, revealing their origin in the innermost jet or accretion disk.

Contribution

It introduces a novel geometric method using Gaia proper motions and photometry to localize optical flares in blazars without relying on multi-wavelength data or models.

Findings

Most flares cause the optical centroid to shift upstream or downstream.

Optical flares are localized within <1 mas of VLBI positions.

Method provides an independent spatial anchor for high-energy flare localization.

Abstract

When blazars flare, their optical position moves. We show this by combining Gaia DR3 proper motions with epoch photometry for blazars with strong optical jet emission. In 60 of 74 sources with significant proper motion, rising flux drives the centroid upstream while fading flux drives it downstream - a near-universal pattern captured by a simple two-component model of constant extended emission and a flaring region. Using this connection, we geometrically localize the optical flares to within <1 mas of the VLBI position - a few parsecs at typical blazar distances - placing them in the innermost jet or accretion disk. This purely geometric method requires no multi-wavelength correlations or model-dependent assumptions, and provides an independent spatial anchor for localizing higher-energy flares. Per-epoch astrometry from Gaia DR4 is set to tighten our constraints even further.