Hard X-ray imaging by Solar Orbiter STIX: challenges, methods and results

The Spectrometer/Telescope for Imaging X-rays (STIX) performs measurements of the X-ray radiation emitted by bremsstrahlung processes during solar flares. STIX imaging is based on a bi-grid system that modulates the incoming photon flux and produces 30 Moiré patterns. By measuring amplitude and phase of the patterns it is possible to obtain the values of amplitude and phase of as many Fourier components of the X-ray source, named visibilities. Hence, the imaging problem for STIX consists in reconstructing the emitted X-ray photon flux from a sparse sampling of its Fourier transform. This problem is particularly challenging due to the limited number of visibilities measured by the instrument, almost one order of magnitude lower w.r.t. that provided by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI).

In this talk we describe the state-of-the-art of the methods developed for solving the STIX imaging problem. Specifically, we focus on improvements of the imaging techniques that been recently implemented, such as the new version of the forward-fitting method based on Particle Swarm Optimization (PSO). We compare the results obtained with the different methods on several events. We also validate the STIX reconstructions by comparing them to the UV maps provided by the Atmospheric Imaging Assembly on board the Solar Dynamic Observatory (SDO/AIA) and by the Extreme Ultraviolet Imager on board Solar Orbiter (EUI).