Using Gaia and synthetic photometry for the absolute flux calibration of planetary cameras: The case of BepiColombo/SIMBIO-SYS

¹ INAF Astronomical Observatory of Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
² INAF Osservatorio di Astrofisica e Scienza dello Spazio, via Gobetti 93/3, 40129 Bologna, Italy
³ Dipartimento di Fisica e Astronomia “Augusto Righi”, Alma Mater Studiorum, Università di Bologna, via Gobetti 93/2, 40129 Bologna, Italy

^★ Corresponding author; giovanni.munaretto@inaf.it

Received: 23 July 2024
Accepted: 10 November 2024

Abstract

Context. The absolute flux calibration of a planetary camera is pivotal for a quantitative analysis of the brightness that is reflected by a celestial body to a) characterise its microphysical properties, b) analyse changes caused by exogenic or endogenic activity, and c) produce high-quality image mosaics to understand the geology of the body. This is usually done by observing a few standard stars. We propose an alternative method that relies on the Gaia catalogue and consider the Spectrometer and Imagers for Mpo Bepicolombo Integrated Observatory SYStem (SIMBIO-SYS) suite of imagers on board BepiColombo, which is currently flying towards planet Mercury, as a test case. We discuss the advantages of this method and its implications for the future exploration of Mercury, as well as the applicability to other cameras.

Aims. We evaluate whether Gaia low-resolution spectra (XPSP) are suitable to supply a set of reference stars for an absolute calibration of a planetary camera. We assess the performances of this approach and discuss its advantages with respect to more traditional methods. While this was validated for common astronomical facilities, it has never been used for planetary cameras.

Methods. We used synthetic photometry from Gaia low-resolution (BP/RP) spectra to produce magnitudes in all the passbands of SIMBIO-SYS. We used a set of very reliable spectrophotometric standard stars to correct small residual systematics that affect externally calibrated BP/RP spectra, and we thus defined the SIMBIO-SYS photometric system tied to the flux scale of the CALSPEC spectrophotometric library. We evaluated the uncertainties on the final calibration of this photometric system by assessing the accuracy (average of the residuals) and precision (standard deviation of the residuals) of the reproduced magnitudes of stars in the CALSPEC spectrophotometric library.

Results. We find an accuracy better than 0.1% and a precision ranging from 0.4% to 1.0%, depending on the considered passband, in the magnitude and colour ranges that are relevant for stars that can be used as photometric standards for the SIMBIO-SYS instrument.

Conclusions. Our results imply an improvement in the flux uncertainty through the absolute calibration of a factor 2–12 with respect to pre-existing imaging data of Mercury, and of a factor 2–7 with respect to the Colour and Stereo Surface Imaging System (CaSSIS), which is a similar camera with the same detector that orbits Mars, for which the absolute calibration details are available in the literature. In the context of the future exploration of Mercury, these improvements imply an unprecedented sensitivity of the SIMBIO-SYS instrument that will provide a novel view of the present-day surface activity on Mercury and of the photometric properties of the Hermean surface.