Using the STIX background detector as a proxy for GOES

Context. The Spectrometer/Telescope for Imaging X-Rays (STIX) on board Solar Orbiter was designed to observe solar flares in the X-ray range of 4−150 keV, providing spectral, temporal, and spatial information. Besides 30 imaging detectors, STIX has two additional detectors: the coarse flare locator (CFL) and the background (BKG) detector, which are used in the present study. Flares observed from Earth are classified using their peak X-ray flux observed by the Geostationary Operational Environmental Satellite (GOES) instruments. Given to the Solar Orbiter mission design, roughly half of all flares observed by STIX are located on the backside of the Sun. These flares lack a GOES-class classification. Aims. In this paper, we describe the calibration of the BKG detector aperture sizes. Using the calibrated measurements of the BKG detector, we explore the relationship between the peak flux for flares jointly observed by STIX and GOES. This allows us to estimate the GOES flare classes of backside flares using STIX measurements. Methods. We looked at the 500 largest flares observed by both STIX and GOES in the time range February 2021 to April 2023. The aperture size calibration was done by comparing 4−10 keV counts of the BKG detector with the CFL measurements. In a second step, we correlated the calibrated STIX BKG peak flux with the GOES peak flux for individual flares. Results. We calibrated the BKG detector aperture sizes of STIX, which are now ready to be implemented into the ground-software (GSW) of STIX. Furthermore, we showed that for the larger flares (C class and above) a close power law fit exists between the STIX BKG and GOES peak flux, with a Pearson correlation coefficient of 0.97. This correlation provides a GOES proxy with a one sigma uncertainty of ≈11%. We were able to show that the BKG detector can reliably measure a broad range of GOES flare classes from roughly B5 up to at least X85 (assuming a radial distance of 1 AU). This makes it an interesting detector-concept for future space weather missions. Thus far, the largest flare observed by STIX to date is an estimated X16.5 ± 1.8 backside flare of May 20, 2024.