Spectrum of the estimate X5-class flare SOL20230103 recorded by the imaging detectors of STIX. The spectrum is integrated over a time range of 10 s between 06:26:50–06:27:00 UTC (Earth time). The measured spectrum is the same in both panels, only the fitting model is different: for the top panel, a photon model including an isothermal model (red), a nonthermal model (green), and an albedo component (gray) is fit. The total fit is given in black. In the bottom panel, we used the same components for the photon model together with a second thermal component (orange). Below each spectrum, the residuals (data minus model divided by the uncertainty on the data points) and the χ² are given. The green bar indicates the energy range used for fitting.