Superhot (> 30 MK) flare observations with STIX: Joint spectral fitting

¹ University of Applied Sciences and Arts Northwestern Switzerland Bahnhofstrasse 6 5210 Windisch, Switzerland
² Institute for Particle Physics and Astrophysics, ETH Zürich Rämistrasse 101 8092 Zürich, Switzerland
³ School of Physics & Astronomy, University of Glasgow University Avenue Glasgow G12 8QQ, UK
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: 12 June 2025
Accepted: 9 December 2025

Abstract

Context. Spectroscopic analysis of large flares (> X1) in the hard X-ray (HXR) range offers unique insights into the hottest (> 30 MK) flare plasma, the so-called superhot thermal component. To manage the high count rates in large flares, an attenuator is typically placed in front of the HXR detectors. However, this significantly limits the spectral diagnostic capabilities at lower energies, and consequently, it restricts the analysis of the lower temperatures in flares.

Aims. The Spectrometer/Telescope for Imaging X-rays (STIX) on board the Solar Orbiter mission was designed to observe solar flares in HXRs. The imaging detectors use an attenuator during periods of high flux. In contrast, the background (BKG) detector of STIX is never covered by the attenuator and is therefore dedicated to measuring the unattenuated flux using differently sized apertures placed in front of the detector. We aim to demonstrate that joint spectral fitting using different detector configurations of STIX allows us to reliably diagnose both the hot and the superhot components in large flares.

Methods. We jointly fit the HXR spectra of the STIX BKG detector and the STIX imaging detectors using the SUNKIT-SPEX software package to determine the spectral parameters of both the hot and superhot thermal components in solar flares.

Results. Using joint fitting on 32 STIX flares, we corroborated that for GOES X-class flares the HXR spectrum is better represented by two thermal components instead of an isothermal component. At the temperature peak time, the superhot HXR flux above ∼15 keV is typically stronger than the hot HXR flux. The GOES long-wavelength channel is dominated by the hot component with a superhot contribution up to 10%.

Conclusions. This paper demonstrates that joint spectral fitting of the same detector type with different attenuation schemes is a simple and powerful method of monitoring multithermal flare plasma.