Using Gaia for the flux calibration of planetary cameras: the BepiColombo/SIMBIO-SYS case

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−2 sr−1 nm−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 ≃ 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≤ λ ≤ 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 & Methods. We obtained synthetic photometry in the SIMBIO-SYS photometric system by convolving both XP spectra and well calibrated spectra (hereafter referred as “reference spectra”) 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.

 

Figure 1.  Difference between synthetic magnitudes computed from the spectra of the adopted reference set of spectrophotometric standards (mag_ref) and those computed from Gaia XP spectra and corrected for systematics (mag^corr_XP) 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 ∆mag = ±0.05 range. The mean and standard deviation of the magnitude difference for the 37 CALSPEC stars having G<10.5 is reported in each panel.

We applied corrections to the original raw XP magnitudes to remove small residual systematics with respect to the CALSPEC reference flux scale. In the following we only refer to these as corrected XP magnitudes.

Results & Discussion. Differences between the synthetic magnitudes computed from the reference spectra of the spectrophotometric standards (mag_ref) and the corresponding corrected XP magnitudes (mag^corr_XP) as a function of G magnitude and for all the STC and HRIC filters are shown in Fig. 1. We evaluate the average residual (i.e., the accuracy) and their standard deviation (i.e., the precision), on the CALSPEC standards. The  mean difference is < 0.001 mag, hence the XP magnitudes in the SIMBIO-SYS system reproduces the CALSPEC photometry with a mean accuracy better than 0.1%. The standard deviation depends on the filter, ranging from 0.004 to 0.010 mag. Hence the corrected XP magnitudes in the SIMBIO-SYS system reproduces the CALSPEC flux scale with a precision < 1%. The calibrated SIMBIO-SYS photometric system is now defined by the XP magnitudes in the SIMBIO-SYS passbands of the standard stars in the SIMBIO-SYS selected fields. To conclude, we show that the availability of high photometric quality Gaia stars can be exploited for providing accurate (below 1%) radiometric calibrations of planetary cameras. For SIMBIO-SYS, in particular, such highly accurate photometry will allow to improve the quality of image mosaics and the quantitative analysis of possible current ongoing surface activity on Mercury, for which SIMBIO-SYS will provide new data after a timespan longer than a decade. Finally, the methodology presented in this paper can in principle be used and evaluated also for other planetary cameras that will explore the Solar System in the next years.

Acknowledgements. MM, MB and PM acknowledge the financial support to activities related to the ESA/mission by the Italian Space Agency (ASI) through contract 2018-24-HH.0 and its addendum 2018-24-HH.1-2022 to the National Institute for Astrophysics (INAF). The study has been supported by the Italian Space Agency (ASI-INAF agreement no. 2020-17-HH.0).

References

Brown, A. G. A. 2021, ARA&A, 59, 59

Gaia Collaboration, Montegriffo, P., Bellazzini, M., et al. 2023a, A&A, 674, A33

Gaia Collaboration, Prusti, T., de Bruijne, J. H. J., et al. 2016, A&A, 595, A1

Gaia Collaboration, Vallenari, A., Brown, A. G. A., et al. 2023b, A&A, 674, A1

De Angeli, F., Weiler, M., Montegriffo, P., et al. 2023, A&A, 674, A2

Montegriffo, P., De Angeli, F., Andrae, R., et al. 2023, A&A, 674, A3

Bohlin, R. C. 2020, in IAU General Assembly, 449–453

Pancino, E., Sanna, N., Altavilla, G., et al. 2021, MNRAS, 503, 3660

Martin, N. F., Starkenburg, E., Yuan, Z., et al. 2023, arXiv e-prints, arXiv:2308.01344