Spectral component imaging of solar X-ray flares

¹ University of Applied Sciences and Arts Northwestern Switzerland, Bahnhofstrasse 6, 5210 Windisch, Switzerland
² ETH Zürich, Rämistrasse 101, 8092 Zürich, Switzerland
³ MIDA, Dipartimento di Matematica, Università di Genova, Via Dodecaneso 35, 16146 Genova, Italy
⁴ Space Sciences Laboratory, University of California, 7 Gauss Way, 94720 Berkeley, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 23 September 2025
Accepted: 14 November 2025

Abstract

Context. Solar hard X-ray (HXR) observations provide diagnostics of the hottest plasmas and of nonthermal electron populations present during solar flares and coronal mass ejections. HXR images of specific energy ranges often contain overlapping contributions of these components, complicating their interpretation. This is even more challenging as HXR imagers generally use an indirect imaging system.

Aims. Our work aims to separately image individual spectral components, such as thermal loops, superhot sources, and nonthermal footpoint sources, rather than obtaining images of specific energy ranges that show a combination of all components.

Methods. We introduced a new method called “spectral component imaging” and applied it to observations provided by the Spectrometer/Telescope for Imaging X-rays (STIX) aboard Solar Orbiter. First, the flare integrated HXR spectrum was fit with individual spectral components to get the relative contributions (“weights”) of each component in each native STIX energy channel. In a second step, a set of linear equations was created based on these weights and the observed, energy-dependent STIX visibilities. The visibilities of the individual spectral components were derived by means of a linear least-squares approach and were subsequently utilized for image reconstructions.

Results. We demonstrate the effectiveness of spectral component imaging on four different flares observed by STIX. This method provides powerful diagnostics, particularly for flares with hot and superhot components, allowing us to spatially separate these two thermal components. We applied our methodology to the nonthermal peak of the X7.1 flare SOL2024-10-01, and we find that the superhot component is located 4.8 Mm away from the hot thermal loops. The thermal energy of the superhot component is approximately 20% of the energy content of the hot component, highlighting the significance of superhot components in the total flare energy budget.

Conclusions. Spectral component imaging provides a powerful tool to image individual spectral components (i.e., thermal and nonthermal X-ray sources), rather than creating images over fixed energy ranges. Because there is no need to select an energy range, spectral component imaging has the potential to automate the image reconstruction process and to establish a robust STIX image database once the spectral components have been defined.