Gaia Early Data Release 3: Photometric content and validation (2012.01916v1)

Abstract: Gaia Early Data Release 3 contains astrometry and photometry results for about 1.8 billion sources based on observations collected by the ESA Gaia satellite during the first 34 months of operations. This paper focuses on the photometric content, describing the input data, the algorithms, the processing, and the validation of the results. Particular attention is given to the quality of the data and to a number of features that users may need to take into account to make the best use of the EDR3 catalogue. The treatment of the BP and RP background has been updated to include a better estimation of the local background, and the detection of crowding effects has been used to exclude affected data from the calibrations. The photometric calibration models have also been updated to account for flux loss over the whole magnitude range. Significant improvements in the modelling and calibration of the point and line spread functions have also helped to reduce a number of instrumental effects that were still present in DR2. EDR3 contains 1.806 billion sources with G-band photometry and 1.540 billion sources with BP and RP photometry. The median uncertainty in the G-band photometry, as measured from the standard deviation of the internally calibrated mean photometry for a given source, is 0.2 mmag at magnitude G=10 to 14, 0.8 mmag at G=17, and 2.6 mmag at G=19. The significant magnitude term found in the Gaia DR2 photometry is no longer visible, and overall there are no trends larger than 1 mmag/mag. Using one passband over the whole colour and magnitude range leaves no systematics above the 1% level in magnitude in any of the bands, and a larger systematic is present for a very small sample of bright and blue sources. A detailed description of the residual systematic effects is provided. Overall the quality of the calibrated mean photometry in EDR3 is superior with respect to DR2 for all bands.

• The paper introduces advanced calibration methods that reduce systematic errors in Gaia EDR3 photometry, achieving median uncertainties as low as 0.2 mmag for bright sources.
• It details refined background estimation and crowding techniques that mitigate contamination from stray light and neighboring sources.
• The findings enhance photometric accuracy and support future astrophysical research and precision cosmological surveys.

Overview of Gaia Early Data Release 3: Photometric Content and Validation

The paper "Gaia Early Data Release 3: Photometric content and validation," presented by Riello and collaborators, provides an in-depth examination of the photometric data contained in the Gaia Early Data Release 3 (EDR3). The document details the methodologies used for processing, calibrating, and validating the photometric data, alongside improvements made from Data Release 2 (DR2) and challenges encountered.

Gaia EDR3 publishes photometric observations for approximately 1.8 billion celestial sources based on data accumulated by the European Space Agency's Gaia satellite over the initial 34 months of its mission. A critical focus of the paper is to present enhancements in photometry data processing methods, including the refinement of flux calibration for blue and red photometers (BP and RP) and the adjustment in photometric calibration models to address instrumental effects.

Key Improvements in Photometric Processing

EDR3 introduces significant advancements in the processing pipeline that improve the quality of the Gaia photometry:

1. Enhanced Calibration Models: EDR3 adopts a new calibration strategy that integrates improved models for flux variations and instrumental effects across the magnitude range. This ensures reductions in systematic errors and accounts for dependencies on source color and scan angles.
2. Flux Loss and Crowding Effects: The background estimation for the BP and RP fluxes includes precise local background assessment and employs crowding evaluation techniques to exclude contaminated data from calibration processes. These improvements mitigate previous constraints caused by stray light and unidentified neighboring sources.
3. Photometric Uncertainty Reductions: The reduction in uncertainty in the $G$-band photometry is notable, with calculated median uncertainties achieving 0.2 mmag for sources between $G = 10$ to $14$, and extending to 2.6 mmag at $G \approx 19$. This reflects improvements in point-spread-function modeling and optimized photometric calibrations.
4. Addressing Prior Observational Artefacts: Noteworthy is the approach to damping down artefacts, such as those affecting earlier releases, through meticulous validation that precluded synchronization artifacts and ensured spatial uniformity in photometry.

Evaluation and Known Issues

The paper carefully outlines known issues with the Gaia EDR3 photometric data and addresses possible mitigations. An identified concern pertains to the overestimation of mean BP flux for faint red sources due to a predefined flux detection threshold, which can bias results toward brighter values at the faint end. Another issue highlighted is with missing SSCs, which affects color determination, necessitating the provision of supplemental calibration data in a separate repository for affected observations.

The documentation further identifies systematic deviations in $G$-band photometry for exceptionally blue and bright sources, suggesting a possible incomplete alignment of instrument response functions that still need addressing.

Implications and Future Work

The advancements presented in EDR3 bolster the precision and accuracy of Gaia's photometric data, thus supporting a wide range of astrophysical applications, from the paper of stellar populations to cosmological surveys. The paper underscores the importance of continuous refinement in calibration techniques to harness Gaia's full potential in providing high-fidelity astronomical data.

Future Gaia releases (e.g., DR4 and beyond) are likely to build upon these systematic enhancements, incorporating lessons learned from EDR3. As the dataset and accompanying calibrations mature, greater emphasis can be placed on secondary science applications, such as time variability studies and extended source analyses.

In summary, Gaia EDR3 provides a notable leap in the quality of photometric data, leveraging technical advancements and comprehensive validations to support cutting-edge astronomical research. As the Gaia mission progresses, further efforts will remain focused on improving calibrations, resolving residual systematic errors, and maximizing the scientific outputs of this global astrometric observatory.