Gaia Data Release 1 Lindegren, L; Lammers, U; Bastian, U ...
Astronomy and astrophysics (Berlin),
11/2016, Letnik:
595
Journal Article
Recenzirano
Odprti dostop
Context. Gaia Data Release 1 (DR1) contains astrometric results for more than 1 billion stars brighter than magnitude 20.7 based on observations collected by the Gaia satellite during the first 14 ...months of its operational phase. Aims. We give a brief overview of the astrometric content of the data release and of the model assumptions, data processing, and validation of the results. Methods. For stars in common with the Hipparcos and Tycho-2 catalogues, complete astrometric single-star solutions are obtained by incorporating positional information from the earlier catalogues. For other stars only their positions are obtained, essentially by neglecting their proper motions and parallaxes. The results are validated by an analysis of the residuals, through special validation runs, and by comparison with external data. Results. For about two million of the brighter stars (down to magnitude ~11.5) we obtain positions, parallaxes, and proper motions to Hipparcos-type precision or better. For these stars, systematic errors depending for example on position and colour are at a level of + or - 0.3 milliarcsecond (mas). For the remaining stars we obtain positions at epoch J2015.0 accurate to ~10 mas. Positions and proper motions are given in a reference frame that is aligned with the International Celestial Reference Frame (ICRF) to better than 0.1 mas at epoch J2015.0, and non-rotating with respect to ICRF to within 0.03 mas yr super(-1). The Hipparcos reference frame is found to rotate with respect to the Gaia DR1 frame at a rate of 0.24 mas yr super(-1). Conclusions. Based on less than a quarter of the nominal mission length and on very provisional and incomplete calibrations, the quality and completeness of the astrometric data in Gaia DR1 are far from what is expected for the final mission products. The present results nevertheless represent a huge improvement in the available fundamental stellar data and practical definition of the optical reference frame.
Gaia Data Release 1 Mignard, F; Klioner, S; Lindegren, L ...
Astronomy and astrophysics (Berlin),
11/2016, Letnik:
595
Journal Article
Recenzirano
Odprti dostop
Context. As part of the data processing for Gaia Data Release 1 (Gaia DR1) a special astrometric solution was computed, the so-called auxiliary quasar solution. This gives positions for selected ...extragalactic objects, including radio sources in the second realisation of the International Celestial Reference Frame (ICRF2) that have optical counterparts bright enough to be observed with Gaia. A subset of these positions was used to align the positional reference frame of Gaia DR1 with the ICRF2. Although the auxiliary quasar solution was important for internal validation and calibration purposes, the resulting positions are in general not published in Gaia DR1. Aims. We describe the properties of the Gaia auxiliary quasar solution for a subset of sources matched to ICRF2, and compare their optical and radio positions at the sub-mas level. Methods. Descriptive statistics are used to characterise the optical data for the ICRF sources and the optical-radio differences. The most discrepant cases are examined using online resources to find possible alternative explanations than a physical optical-radio offset of the quasars. Results. In the auxiliary quasar solution 2191 sources have good optical positions matched to ICRF2 sources with high probability. Their formal standard errors are better than 0.76 milliarcsec (mas) for 50% of the sources and better than 3.35 mas for 90%. Optical magnitudes are obtained in Gaia's unfiltered photometric G band. The Gaia results for these sources are given as a separate table in Gaia DR1. The comparison with the radio positions of the defining sources shows no systematic differences larger than a few tenths of a mas. The fraction of questionable solutions, not readily accounted for by the statistics, is less than 6%. Normalised differences have extended tails requiring case-by-case investigations for around 100 sources, but we have not seen any difference indisputably linked to an optical-radio offset in the sources. Conclusions. With less than a quarter of the data expected from the nominal mission it has been possible to obtain positions at the sub-mas level for most of the ICRF sources having an optical counterpart brighter than 20.5 mag.
The ESA space astrometry mission Gaia is designed to measure the positions, parallaxes and proper motions of more than one billion stars of our Galaxy brighter than 20 magnitude. The expected ...astrometric accuracies are in the range 7–25
μas for the point-like sources down to 15
mag. To achieve this staggering performance, several issues of highly accurate metrology must be solved and implemented on the spacecraft. In this paper, I discuss first some constraining principles to perform global astrometry in space, and then a couple of examples of the metrological issues currently faced by the design or to be encountered during the operations. One concerns the optimisation of the sky coverage with a scanning instrument like Gaia, and the other deals with the instrument stability and how the stringent requirements are met in practice.
Gaia Data Release 1 Fabricius, C; Bastian, U; Portell, J ...
Astronomy and astrophysics (Berlin),
11/2016, Letnik:
595
Journal Article
Recenzirano
Odprti dostop
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 super(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.
Abstract
Combining the exquisite angular resolution of Gaia with optical light curves and WISE photometry, the Gaia Gravitational Lenses group (GraL) uses machine-learning techniques to identify ...candidate strongly lensed quasars, and has confirmed over two dozen new strongly lensed quasars from the Gaia Data Release 2. This paper reports on the 12 quadruply imaged quasars identified by this effort to date, which is a ∼20% increase in the total number of confirmed quadruply imaged quasars. We discuss the candidate selection, spectroscopic follow-up, and lens modeling. We also report our spectroscopic failures as an aid for future investigations.
Short-arc orbit determination is crucial when an asteroid is first discovered. In these cases usually the observations are so few that the differential correction procedure may not converge. We ...developed an initial orbit computation method, based on systematic ranging, which is an orbit determination technique that systematically explores a raster in the topocentric range and range-rate space region inside the admissible region. We obtained a fully rigorous computation of the probability for the asteroid that could impact the Earth within a few days from the discovery without any a priori assumption. We tested our method on the two past impactors, 2008 TC3 and 2014 AA, on some very well known cases, and on two particular objects observed by the European Space Agency Gaia mission.
Aims. We compare stellar catalogues with position and proper motion components using a decomposition on a set of orthogonal vector spherical harmonics. We aim to show the theoretical and practical ...advantages of this technique as a result of invariance properties and the independence of the decomposition from a prior model. Methods. We describe the mathematical principles used to perform the spectral decomposition, evaluate the level of significance of the multipolar components, and examine the transformation properties under space rotation. Results. The principles are illustrated with a characterisation of systematic effects in the FK5 catalogue compared to Hipparcos and with an application to extraction of the rotation and dipole acceleration in the astrometric solution of QSOs expected from Gaia.
The GAIA astrometric mission has recently been approved as one of the next two "cornerstones"of ESA's science programme, with a launch date target of not later than mid-2012. GAIA will provide ...positional and radial velocity measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars throughout our Galaxy (and into the Local Group), amounting to about 1 percent of the Galactic stellar population. GAIA's main scientific goal is to clarify the origin and history of our Galaxy, from a quantitative census of the stellar populations. It will advance questions such as when the stars in our Galaxy formed, when and how it was assembled, and its distribution of dark matter. The survey aims for completeness to $V=20$ mag, with accuracies of about 10 μas at 15 mag. Combined with astrophysical information for each star, provided by on-board multi-colour photometry and (limited) spectroscopy, these data will have the precision necessary to quantify the early formation, and subsequent dynamical, chemical and star formation evolution of our Galaxy. Additional products include detection and orbital classification of tens of thousands of extra-Solar planetary systems, and a comprehensive survey of some $10^5{-}10^6$ minor bodies in our Solar System, through galaxies in the nearby Universe, to some 500 000 distant quasars. It will provide a number of stringent new tests of general relativity and cosmology. The complete satellite system was evaluated as part of a detailed technology study, including a detailed payload design, corresponding accuracy assesments, and results from a prototype data reduction development.
The Gaia cornerstone mission of the European Space Agency, scheduled for launch in mid-2013, is expected to produce a breakthrough in our understanding of the Galaxy, providing a full ...characterisation of all sources by extremely accurate astrometry and complementary spectral data. The continuous scanning of the sky and the fully automated selection of the sources also ensure that nearly all Solar System objects brighter than V=20 will be observed by Gaia. We describe the expectations of the mission in Solar System science and the peculiar properties of asteroid data, requiring appropriate data reduction procedures currently being implemented. Recent estimates of the number of observed sources and other statistical properties of the sample are presented.
► The number of asteroids Gaia will detect. ► The accuracy budget is explored for different object categories. ► The on-board windowing strategy.
Context.
Since July 2014, the
Gaia
mission of the European Space Agency has been surveying the entire sky down to magnitude 20.7 in the visible. In addition to the millions of daily observations of ...stars, thousands of Solar System objects (SSOs) are observed. By comparing their positions, as measured by
Gaia
, to those of known objects, a daily processing pipeline filters known objects from potential discoveries. However, owing to
Gaia
’s specific observing mode, which follows a predetermined scanning law designed for stars as “fixed” objects on the celestial sphere, potential newly discovered moving objects are characterized by very few observations, which are acquired over a limited time. Furthermore, these objects cannot be specifically targeted by
Gaia
itself after their first detection. This aspect was recognized early on in the design of the
Gaia
data processing.
Aims.
A daily processing pipeline dedicated to these candidate discoveries was set up to release calls for observations to a network of ground-based telescopes. Their aim is to acquire follow-up astrometry and to characterize these objects.
Methods.
From the astrometry measured by
Gaia
, preliminary orbital solutions are determined, allowing us to predict the position of these potentially newly discovered objects in the sky while accounting for the large parallax between
Gaia
and the Earth (separated by 0.01 au). A specific task within the
Gaia
Data Processing and Analysis Consortium has been responsible for the distribution of requests for follow-up observations of potential
Gaia
SSO discoveries. Since late 2016, these calls for observations (nicknamed “alerts”) have been published via a Web interface with a quasi-daily frequency, together with observing guides, which is freely available to anyone worldwide.
Results.
Between November 2016 and the end of the first year of the extended mission (July 2020), over 1700 alerts were published, leading to the successful recovery of more than 200 objects. Among them, six have a provisional designation assigned with the
Gaia
observations; the others were previously known objects with poorly characterized orbits, precluding identification at the time of
Gaia
observations. There is a clear trend for objects with a high inclination to be unidentified, revealing a clear bias in the current census of SSOs against high-inclination populations.
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