Gaia Data Release 1 Summary of the astrometric, photometric, and survey properties

Context. At about 1000 days after the launch of Gaia we present thefirst Gaia data release, Gaia DR1, consisting of astrometry andphotometry for over 1 billion sources brighter than magnitude 20.7.Aims. A summary of Gaia DR1 is presented along with illustrations of thescientific quality of the data, followed by a discussion of thelimitations due to the preliminary nature of this release.Methods. The raw data collected by Gaia during the first 14 months ofthe mission have been processed by the Gaia Data Processing and AnalysisConsortium (DPAC) and turned into an astrometric and photometriccatalogue.Results. Gaia DR1 consists of three components: a primary astrometricdata set which contains the positions, parallaxes, and mean propermotions for about 2 million of the brightest stars in common with theHIPPARCOS and Tycho-2 catalogues - a realisation of the Tycho-GaiaAstrometric Solution (TGAS) - and a secondary astrometric data setcontaining the positions for an additional 1.1 billion sources. Thesecond component is the photometric data set, consisting of mean G-bandmagnitudes for all sources. The G-band light curves and thecharacteristics of similar to 3000 Cepheid and RR Lyrae stars, observedat high cadence around the south ecliptic pole, form the thirdcomponent. For the primary astrometric data set the typical uncertaintyis about 0.3 mas for the positions and parallaxes, and about 1 masyr(-1) for the proper motions. A systematic component of similar to 0.3mas should be added to the parallax uncertainties. For the subset ofsimilar to 94 000 HIPPARCOS stars in the primary data set, the propermotions are much more precise at about 0.06 mas yr(-1). For thesecondary astrometric data set, the typical uncertainty of the positionsis similar to 10 mas. The median uncertainties on the mean G-bandmagnitudes range from the mmag level to similar to 0.03 mag over themagnitude range 5 to 20.7.Conclusions. Gaia DR1 is an important milestone ahead of the next Gaiadata release, which will feature five-parameter astrometry for allsources. Extensive validation shows that Gaia DR1 represents a majoradvance in the mapping of the heavens and the availability of basicstellar data that underpin observational astrophysics. Nevertheless, thevery preliminary nature of this first Gaia data release does lead to anumber of important limitations to the data quality which should becarefully considered before drawing conclusions from the data.