GaiaData Release 1

Abstract

Context. The first data release from the Gaia mission contains accurate positions and magnitudes for more than a billion sources, and proper motions and parallaxes for the majority of the 2.5 million Hipparcos and Tycho-2 stars. Aims. We describe three essential elements of the initial data treatment leading to this catalogue: the image analysis, the construction of a source list, and the near real-time monitoring of the payload health. We also discuss some weak points that set limitations for the attainable precision at the present stage of the mission. Methods. Image parameters for point sources are derived from one-dimensional scans, using a maximum likelihood method, under the assumption of a line spread function constant in time, and a complete modelling of bias and background. These conditions are, however, not completely fulfilled. The Gaia source list is built starting from a large ground-based catalogue, but even so a significant number of new entries have been added, and a large number have been removed. The autonomous onboard star image detection will pick up many spurious images, especially around bright sources, and such unwanted detections must be identified. Another key step of the source list creation consists in arranging the more than 10 10 individual detections in spatially isolated groups that can be analysed individually. Results. Complete software systems have been built for the Gaia initial data treatment, that manage approximately 50 million focal plane transits daily, giving transit times and fluxes for 500 million individual CCD images to the astrometric and photometric processing chains. The software also carries out a successful and detailed daily monitoring of Gaia health.

Contrast sensitivities in the Gaia Data Release 2

“…This could be due to spurious detections, which are well known to be present around bright stars (Sect. 6.3 of Fabricius et al 2016), or underestimated errors for sources near bright targets, so that actual companions show spurious parallaxes or proper motions sufficiently different from the target to not be removed. By using the maximum source density (including remaining physical companions), we effectively assume that the source density is constant within the separation of maximum background density, and that the reason for the lower observed density is the reduced sensitivity at close separation.…”

The Precious Set of Radio-optical Reference Frame Objects in the Light of Gaia DR2 Data

“…3 shows a few examples of DES images, which indicate problematic cases for optical astrometry. Any deviation from a nominal star-like image can significantly perturb the Gaia-determined photocenter, because a single template line spread function was applied for each CCD and each gate (Fabricius et al 2016) in the low-level pipeline processing. These perturbations are not always captured by the χ 2 -based residual statistic, apparently.…”

“…The nearby AGNs are often associated with their elliptical or spiral host galaxies. The extended substrate image perturbs Gaia astrometry at the centroid fitting level, as the latter procedure was designed for unresolved, point-like sources in Gaia DR1 and DR2 (Fabricius et al 2016). According to the "fundamental plane" relation proposed by Hamilton et al (2008), the magnitude difference in V between the host galaxy and the nucleus is larger for luminous nuclei.…”

NGTS-7Ab: an ultrashort-period brown dwarf transiting a tidally locked and active M dwarf

“…Fig. 7 in Fabricius et al 2016), we expect only minimal contamination between the stars in the Gaia G band photometry. Consequently, we use the Gaia G band photometry in our analysis.…”