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

TL;DR

This paper reports precise Johnson-Kron-Cousins BVRI photometry of 228 candidate Gaia flux standards, crucial for calibrating Gaia data and improving the accuracy of astronomical flux measurements.

Contribution

It provides a large, well-calibrated set of ground-based flux standards for Gaia, with detailed comparisons showing about 1% agreement with literature, enhancing calibration accuracy.

Findings

Absolute photometry tied to Landolt standards within 1%

Photometric precision of Gaia data is about 0.1% or better

Comparison with literature shows ~1% agreement

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 $\simeq$1 per cent or better, depending on the photometric band. Extensive comparisons with various literature sources show an overall $\simeq$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.