Using Gaia for the flux calibration of planetary cameras: the BepiColombo/SIMBIO-SYS case
Authors: Munaretto Giovanni , Cremonese Gabriele , Bellazzini Michele , Montegriffo Paolo , Sordo Rosanna , La Grassa Riccardo , Re Cristina , Tullo Adriano , Messineo Maria 
Published: July 2024

Introduction. Flux calibration is a key procedure for the full scientific exploitation of the data acquired by planetary remote-sensing cameras. It is necessary to produce high quality, seamless, global and regional color and monochrome mosaics, which are pivotal for the geologic analysis of any planetary surface. It is also fundamental for the quantitative analysis of surface changes, space weathering effects, and for assessing the photometric properties of planetary regoliths. It consists of converting raw data (Digital Numbers, DN) to absolute physical units (W m&#8722;2 sr&#8722;1 nm&#8722;1 or I/F, i.e. the ratio between observed radiance and the radiance of a 100% lambertian reflector with the Sun and camera orthogonal to the observing surface) and requires sources with accurate spectral irradiances or integrated fluxes (i.e, magnitudes). The ESA-Gaia space mission (Gaia Collaboration et al. 2016) is collecting exquisite astrometry and photometry for about two billion stars brighter than G &#8771; 20.5 since 2014 (Brown 2021). In the latest data release (Gaia DR3, Gaia Collaboration et al. 2023b), very low resolution spectra (XP spectra hereafter) have been released for the first time, for about 220 million sources (De Angeli et al. 2023). Gaia Collaboration et al. (2023a) demonstrated that remarkably accurate and very precise synthetic photometry can be obtained from flux-calibrated (Montegriffo et al. 2023) XP spectra virtually for any passband whose wavelength range is entirely enclosed within 330 nm&#8804; &#955; &#8804; 1050 nm. This opens for the first time the possibility to get precise space-based all-sky photometry for a huge number of stars to calibrate the photometric systems of other surveys in the optical range, operating both from space or from the ground. Indeed, synthetic photometry from XP spectra (XPSP hereafter) has been already used for calibration and validation of various photometric surveys (see, e.g., Martin et al. 2023) and is becoming a fundamental photometric reference in the optical domain. In this paper, we assess the potential of using Gaia XPSP for the absolute flux calibration of a planetary remote sensing camera by taking the SIMBIO-SYS instrument on the ESA/JAXA BepiColombo mission as a test case.Data &amp; Methods. We obtained synthetic photometry in the SIMBIO-SYS photometric system by convolving both XP spectra and well calibrated spectra (hereafter referred as &#8220;reference spectra&#8221;) from three different libraries of spectrophotometric standard stars through the filter passbands, wavelength dependent detector quantum efficiency, and wavelength dependent mirror reflectivity. We considered the Gaia Spectro Photometric Standard Stars (SPSS; Pancino et al. 2021), The Passband Validation Library (PVL; Pancino et al. 2021), and the latest version of the CALSPEC library (Bohlin et al. 2020). The latter is entirely made of space-based spectra and is generally considered as the best reference stellar flux scale. For each standard, we compare the SIMBIO-SYS synthetic photometry calculated from the GAIA XP with the corresponding synthetic photometry coming from the reference spectra.&#160;Figure 1.&#160; Difference between synthetic magnitudes computed from the spectra of the adopted reference set of spectrophotometric standards (magref) and those computed from Gaia XP spectra and corrected for systematics (magcorrXP) for the STC and HRIC filters. Grey triangles are SSP stars, grey square PVL stars and blue circles are CALSPEC stars. The thin horizontal lines enclose the &#8710;mag = &#177;0.05 range. The mean and standard deviation of the magnitude difference for the 37 CALSPEC stars having G