Differential Astrometry with Gaia: Investigating relativistic light deflection close to Jupiter

TL;DR

This paper develops a differential astrometric framework for Gaia to measure relativistic light deflection near Jupiter, demonstrating Gaia's capability to detect such signals with high precision under challenging observational conditions.

Contribution

It introduces a novel differential astrometric model tailored for Gaia, enabling the first detection of relativistic light deflection close to Jupiter's limb with high signal-to-noise ratio.

Findings

Relativistic deflection detected with S/N of 50 near Jupiter

First optical detection of light deflection within 7 arcseconds of Jupiter

Gaia's astrometric performance validated under extreme observational conditions

Abstract

In this paper we develop a differential astrometric framework that is appropriate for a scanning space satellite such as Gaia. We apply it to the first of the GAREQ fields, which is the fruit of dedicated efforts within the Gaia project focused on measuring the relativistic deflection of light close to Jupiter's limb. This provides a preliminary assessment of: a) the observability of the relativistic deflection of light close to Jupiter, and, b) Gaia's astrometric capabilities under extremely difficult conditions such as those around a bright extended object. Inputs to our differential astrometric model are the CCD transit times as measured by the IDU system, transformed to field angles via AGIS geometric calibrations, and the commanded/nominal spacecraft attitude. [abridged] To illustrate the model we analyze Gaia astrometric measurements after their calibration through the latest cyclic EDR3/DR3 processing of the GAREQ event of Feb 2017. We use observations of the closest bright target star successfully observed several times by Gaia in close proximity to Jupiter and surrounding reference stars brighter than G<13 mag in transits leading up to the time of closest approach and on subsequent transits. The relativistic deflection signal is detected with a S/N of 50 at closest approach by the target star. This signal is the combined effect due to Jupiter and the Sun, mainly dominated by Jupiter's monopole, demonstrating Gaia's scientific performance under extreme observational conditions. It is an unprecedented detection for the following reasons: a) closest ever to Jupiter's limb (~7") in the optical, and, b) highest S/N at any wavelength. Finally, this work sets the stage for investigations into disentangling the relativistic quadrupole deflection due to Jupiter with future Gaia astrometric measurements.