Differential astrometry with Gaia

Abstract

Aims. 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 – the Gaia Relativistic Experiment on Jupiter’s quadrupole – which is the fruit of dedicated efforts within the Gala project focused on measuring the relativistic deflection of light close to Jupiter’s limb. This provides a preliminary assessment of the following: 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. Methods. Inputs to our differential astrometric model are the charge-coupled device (CCD) transit times as measured by the intermediate data update (IDU) system, transformed to field angles via astrometric global iterative solution (AGIS) geometric calibrations, and the commanded or nominal spacecraft attitude. Actual attitude rates, including medium and high-frequency effects, were estimated from successive CCD pair observations and used to transfer the field angles onto intermediate tangent planes, finally anchored to a common reference frame by fitting a six-parameter model to a set of suitable reference stars. The best-fit parameters provide the target star’s deflection as a time-varying systematic effect. To illustrate the model, we analyzed Gaia astrometric measurements after their calibration through the latest cyclic early data release EDR3/DR3 processing of the GAREQ event in February 2017. We used 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. Results. The relativistic deflection signal is detected with a signal-to-noise ratio (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 Gala’s scientific performance under extreme observational conditions. It is an unprecedented detection for the following reasons: a) it is the closest ever to Jupiter’s limb (~7″) in the optical and b) the 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.