In-Flight Self-Calibration of the STIX Grid Transmission.

The Spectrometer/Telescope for Imaging X-rays (STIX) on board Solar Orbiter provides imaging information on the X-ray sources in solar flares by modulating the emitted radiation with pairs of grids made of stacked tungsten layers. Optical measurements of the grids were taken on the ground and utilized to derive an initial model for the grid transmission. In this work, we describe an in-flight self-calibration procedure of the STIX grid transmission below 20 keV. This technique utilizes fully-illuminated pixels of the STIX Coarse Flare Locator (CFL) as a total flux monitor. The results show that the newly established transmission is lower than what was derived from the optical measurements. This is due to imperfections of the tungsten layers which reduce the effective slit width. Pre-flight optical measurements only measure the outer layers leaving these imperfections undetected. Using the new calibration, the photon spectra derived from individual sub-collimators are consistent within ∼ 2% between 8 and 20 keV. While fitted flare temperatures do not change with the new calibration, emission measure values increase by ∼ 13% compared to those obtained with the initial grid measurements.