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Register a new userSystematics and accuracy of VLBI astrometry: What can be learned from a comparison with Gaia Data Release 2
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We aim to investigate the overall properties of the ICRF3 with the help of the Gaia Data release 2 (Gaia DR2). This could serve as an external check of the quality of the ICRF3. The radio source positions of the ICRF3 catalog were compared with the Gaia DR2 positions of their optical counterparts at G < 18.7. Their properties were analyzed in terms of the dependency of the quoted error on the number of observations, on the declination, and the global difference, the latter revealed by means of expansions in the vector spherical harmonics. The ICRF3 S/X-band catalog shows a more smooth dependency on the number of observations than the ICRF1 and ICRF2, while the K and X/Ka-band yield a dependency discrepancy at the number of observations of about 50. The rotation of all ICRF catalogs show consistent results, except for the X-component of the X/Ka-band which arises from the positional error in the non-defining sources. No significant glides were found between the ICRF3 S/X-band component and Gaia DR2. However, the K- and X/Ka- band frames show a dipolar deformation in Y-component of +50{mu}as and several quadrupolar terms of 50{mu}as in an absolute sense. A significant glide along Z-axis exceeding 200 {mu}as in the X/Ka-band was also reported. These systematics in the ICRF catalog are shown to be less dependent on the limiting magnitude of the Gaia sample when the number of common sources is sufficient (> 100). The ICRF3 S/X-band catalog shows improved accuracy and systematics at the level of noise floor. But the zonal errors in the X/Ka-band should be noted, especially in the context of comparisons of multi-frequency positions for individual sources.
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Aims. We describe the photometric content of the second data release of the Gaia project (Gaia DR2) and its validation along with the quality of the data. Methods. The validation was mainly carried out using an internal analysis of the photometry. External comparisons were also made, but were limited by the precision and systematics that may be present in the external catalogues used. Results. In addition to the photometric quality assessment, we present the best estimates of the three photometric passbands. Various colour-colour transformations are also derived to enable the users to convert between the Gaia and commonly used passbands. Conclusions. The internal analysis of the data shows that the photometric calibrations can reach a precision as low as 2 mmag on individual CCD measurements. Other tests show that systematic effects are present in the data at the 10 mmag level.
The second Gaia data release is based on 22 months of mission data with an average of 0.9 billion individual CCD observations per day. A data volume of this size and granularity requires a robust and reliable but still flexible system to achieve the demanding accuracy and precision constraints that Gaia is capable of delivering. The internal Gaia photometric system was initialised using an iterative process that is solely based on Gaia data. A set of calibrations was derived for the entire Gaia DR2 baseline and then used to produce the final mean source photometry. The photometric catalogue contains 2.5 billion sources comprised of three different grades depending on the availability of colour information and the procedure used to calibrate them: 1.5 billion gold, 144 million silver, and 0.9 billion bronze. These figures reflect the results of the photometric processing; the content of the data release will be different due to the validation and data quality filters applied during the catalogue preparation. The photometric processing pipeline, PhotPipe, implements all the processing and calibration workflows in terms of Map/Reduce jobs based on the Hadoop platform. This is the first example of a processing system for a large astrophysical survey project to make use of these technologies. The improvements in the generation of the integrated G-band fluxes, in the attitude modelling, in the cross-matching, and and in the identification of spurious detections led to a much cleaner input stream for the photometric processing. This, combined with the improvements in the definition of the internal photometric system and calibration flow, produced high-quality photometry. Hadoop proved to be an excellent platform choice for the implementation of PhotPipe in terms of overall performance, scalability, downtime, and manpower required for operations and maintenance.
Context. Gaia Early Data Release 3 (Gaia EDR3) provides accurate astrometry for about 1.6 million compact (QSO-like) extragalactic sources, 1.2 million of which have the best-quality five-parameter astrometric solutions. Aims. The proper motions of QSO-like sources are used to reveal a systematic pattern due to the acceleration of the solar system barycentre with respect to the rest frame of the Universe. Apart from being an important scientific result by itself, the acceleration measured in this way is a good quality indicator of the Gaia astrometric solution. Methods. The effect of the acceleration is obtained as a part of the general expansion of the vector field of proper motions in Vector Spherical Harmonics (VSH). Vario
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