The Gaia spectrophotometric standard stars survey – IV. Results of the absolute photometry campaign

Abstract

We present Johnson-Kron-Cousins BVRI photometry of 228 candidate spectrophotometric standard stars for the external (absolute) flux calibration of Gaia data. The data were gathered as part of a ten-year observing campaign with the goal of building the external grid of flux standards for Gaia and we obtained absolute photometry, relative photometry for constancy monitoring, and spectrophotometry. Preliminary releases of the flux tables were used to calibrate the first two Gaia releases. This paper focuses on the imaging frames observed in good sky conditions (about 9100). The photometry will be used to validate the ground-based flux tables of the Gaia spectrophotometric standard stars and to correct the spectra obtained in non-perfectly photometric observing conditions for small zeropoint variations. The absolute photometry presented here is tied to the Landolt standard stars system to ≃1 per cent or better, depending on the photometric band. Extensive comparisons with various literature sources show an overall ≃1 per cent agreement, which appears to be the current limit in the accuracy of flux calibrations across various samples and techniques in the literature. The Gaia photometric precision is presently of the order of 0.1 per cent or better, thus various ideas for the improvement of photometric calibration accuracy are discussed.

The Gaia spectrophotometric standard stars survey -- V. Preliminary flux tables for the calibration of Gaia DR2 and (E)DR3

“…We Table 1. List of SPSS included in the V1 and V2 sets along with basic information: the SPSS ID and name; the Gaia EDR3 ID and coordinates (rounded in the pdf table version); the B and V magnitudes from Altavilla et al (2021), except for LTT 377 (SPSS 350), where they are from Koen et al (2010); the spectral type collected from the literature by Pancino et al (2012); the SPSS set in which the star was included, either V1 for Gaia DR2 or V2 for Gaia (E)DR3; the quality flag (0 = Ok, 1 = warning on minor spectral defects, 2 = warning on spectral defects) and the notes. Only the first five lines are shown here; the table is available in its entirety online (see Data Availability Section).…”

“…Overall, we were awarded more than 5 000 observing hours at seven different observing facilities, of which a large fraction was carried out in visitor mode. We carried out three main campaigns: (i) a constancy monitoring campaign, described in details in Marinoni et al (2016); (ii) an absolute photometry campaign, described in details in Altavilla et al (2021); and (iii) a spectro-photometric campaign, for which we present the first results here. We also started a fourth campaign, devoted to the constancy monitoring on longer timescales (about three years) (see Pancino et al 2012, and Section 5.1).…”

“…In a few cases, we observed that some spectra showed a significant slope when compared to the median of the photometric spectra: we included them only when the slope could be successfully removed (within 1% of maximum deviation). In future SPSS releases we will use our absolutely calibrated photometry (Altavilla et al 2021) to validate andif necessary and appropriate -to re-adjust the calibration of spectra taken in non perfectly photometric conditions (see also Section 5.3).…”

“…We also computed the synthetic Johnson-Kron-Cousins UBVRI magnitudes on the SPSS V2 set and compared them with our own absolutely calibrated magnitudes (Altavilla et al 2021) and, for 36 stars, with the original measurements in a series of papers by Landolt and collaborators (Landolt 1983(Landolt , 1992b Cousins magnitudes, we used the passbands by Bessell & Murphy (2012). The choice of the Vega spectrum is crucial, as variations of 1-2% can be easily obtained in the results when using different spectra.…”

Internal calibration of Gaia BP/RP low-resolution spectra

“…1, after an initial preparation step called preprocessing, the internal (or relative) calibration uses only Gaia observations to relate all observations into a common system. After this internal calibration, the external (or absolute) calibration uses a relatively small number of ground-based standard stars (Pancino et al 2012(Pancino et al , 2021Altavilla et al 2021) to get the absolute fluxes and wavelengths. This paper describes the concepts of the internal calibration of Gaia spectrophotometric data, but this is not linked to the particular details used in any of the Fig.…”