Analytical modelling of STIX instrumental background and its application in the spectral and imaging analysis

STIX records X-rays from Solar activity from 4 to 150 keV with 32 detectors, each divided into 12 pixels. Background is obtained using the BKG detector and is known to essentially correspond to the background radioactive sources onboard STIX. This study aims to construct an analytical model of the STIX background based on available measures. The model will allow predicting the background at any time interval of the mission. We analysed 256 background files recorded by STIX from January 2022 to July 2023, and we assume that onboard calibration sources ( ¹³³ Ba) dominate the background. We fitted with an exponential function corresponding to the half-life of ¹³³ Ba to the background in each of the 384 pixels and each of the science energy bins. Interpolated background values were validated against additional files for consistency. These values were then integrated into image reconstruction algorithms to assess variability in source parameters across background approaches. A determined background was also tested in OSPEX to evaluate its impact on spectral fitting. The analysis shows that the STIX background is non-uniform, with pixel-to-pixel variations affecting moiré patterns in weak flares. Our approximation matches background files collected during quiet Sun periods, but it can also differ from them during high activity or perihelion, where onboard files include additional signals. In such cases, our radioactive-source-based background offers a more reliable basis for imaging and spectral analysis than file-based background and enables a higher degree of automation in the analysis. Background analysis from Jan 2022 to Jul 2023 shows that STIX background is usually dominated by ¹³³ Ba over the full energy spectrum. Interpolated background methods improve accuracy during high solar activity or close Sun approaches, where standard methods falter. This enhances X-ray image quality and enables reconstruction from fewer counts, supporting more precise analyses, especially of microflares.