3D reconstruction of thermal hard X-ray sources in solar flares from combined STIX and HXI visibilities

Abstract

Context. The Spectrometer/Telescope for Imaging X-rays (STIX) on board the ESA Solar Orbiter mission and the Hard X-ray Imager (HXI) aboard the Advanced Space-based Solar Observatory (ASO-S) satellite provide for the first time systematic coverage of solar flare hard X-ray sources from different vantage points. This unprecedented configuration enables the reconstruction of the three-dimensional (3D) hard X-ray intensity distribution of the thermal emission in solar flares. Aims. The main objectives of this study are twofold: (1) to perform 3D reconstructions of the thermal hard X-ray source of a solar flare using stereoscopic Fourier data (visibilities) provided by STIX and HXI; and (2) to investigate the evolution in time of the reconstructed 3D source morphology. Methods. The sets of 2D visibilities measured by STIX and HXI represent a sampling of the 3D Fourier transform of the flaring hard X-ray source on two planes orthogonal to the instruments’ lines of sight. Therefore, the 3D reconstruction problem is analogous to the standard 2D imaging problem and can be addressed with similar techniques. In this case, we performed 3D reconstructions by means of the iterative space reconstruction algorithm (ISRA). Results. We consider the SOL2024-10-03T12:12 event observed by STIX and HXI from largely different vantage points separated by 85.7°. We performed 3D reconstructions of the flaring thermal emission at a 10 s cadence, and we determined the X-ray source height and radial velocity over time. As a consistency test, we show that the morphology and location of our 3D reconstructions are consistent with the corresponding 2D images independently obtained from STIX and HXI data. Conclusions. The proposed 3D reconstruction methodology provides reliable results for events with a separation angle close to 90°. However, the dynamic range of the 3D reconstructions is limited by the low number of observed visibilities (as in the 2D case) and by the limited number of vantage points on the flaring events.