The First STIX Coarse Flare Locations

The Spectrometer/Telescope for Imaging X-rays (STIX) on board Solar Orbiter (Krucker et al., 2020) uses an indirect imaging system to measure flare location, size and morphology. Pairs of tungsten grids create Moiré fringes on its coarsely pixelated CdTe detectors. Images are then reconstructed on the ground, using sophisticated imaging algorithms, after the data containing the counts in each pixel for 30 imaging detectors has been download. 

STIX therefore uses a dedicated sub-collimator to estimate a rough (within a few arcminutes), but unambiguous, flare location on board in near real time. The Coarse Flare Locator (CFL) consists of a single grid with a specific pattern which selectively illuminates pixels of a dedicated detector based on the source location. The correlation between the counts in the pixels of this detector, combined with sums of counts from the other detectors, and a look up table of pre-caculated expectations allows the location to be estimated promptly, within the constraints of on board processing. 

Using the downloaded measured counts in each pixel the coarse flare location can also be reconstructed on the ground. This allows for more sophisticated algorithms which require greater computational power than is available on board; greater flexibility as to which time and energy intervals are combined; and more careful background subtraction possible. 

The first estimates of STIX flare locations calculated using the STIX Ground Processing Software (GSW) from data taken during commissioning and subsequent Remote Sensing Checkout Windows are presented here. Comparisons are made to the expected active region and source locations, using data from several other instruments. 

References 

Krucker, S et al. The Spectrometer/Telescope for Imaging X-rays (STIX). Astronomy and Astrophysics, v. 642, Oct. 2020. DOI: 10.1051/0004-6361/201937362.