Imaging from STIX visibility amplitudes

Aims. This study is aimed at providing the first demonstration of STIX Fourier-transform X-ray imaging using semi-calibrated (amplitude-only) visibility data acquired during Solar Orbiter's cruise phase. Methods. We used a parametric imaging approach whereby STIX visibility amplitudes are fitted by means of two non-linear optimization methods: a fast meta-heuristic technique inspired by social behavior and a Bayesian Monte Carlo sampling method that, although slower, provides a better way of quantifying the uncertainties. Results. When applied to a set of solar flare visibility amplitudes recorded by STIX on November 18, 2020, the two parametric methods provide very coherent results. The analysis also demonstrates the ability of STIX to reconstruct data at a high time resolution and, from a spectral viewpoint, it shows the reliability of a double-source scenario that is consistent with a thermal versus nonthermal interpretation. Conclusions. In this preliminary analysis of STIX imaging based only on visibility amplitudes, we formulate the imaging problem as a non-linear parametric issue addressed by means of two high-performance optimization techniques that both show the ability to sample the parametric space in an effective fashion, thus avoiding any local minima.