Observed STIX spectra (black) integrated over 2 s around the second large non-thermal peak of the SOL2024-05-20T05:14L171C109 (estimated-X16.5) flare (05:13:21 – 05:13:23). The summed spectra from the 24 coarsest angular resolution imaging sub-collimators (left) and pixels 2 and 5 of the background detector (BKG; right) are shown. The best fit (green) is derived by applying an iterative fitting method (Stiefel et al. 2025) to both spectra with a model including thermal (red), superhot thermal (yellow), non-thermal thick-target (blue), and albedo (grey) components. The fitted spectral ranges (12 – 100 keV for imaging detectors; 6 – 28 keV for the BKG detector) are denoted by the vertical dotted lines. The lower panels show the fit residuals, whose calculation included a 5% systematic error. The best-fit model parameters are given in the top right, along with their uncertainties.