The Spectrometer/Telescope for Imaging X-rays (STIX)

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1 STIX instrument Scientific Objectives

STIX plays an important role in enabling Solar Orbiter to achieve two of its major science goals: (1) determining the magnetic connection of Solar Orbiter back to the Sun and (2) understanding the acceleration of electrons at the Sun and their transport into interplanetary space. The X-ray measurements made with STIX determine the intensity, spectrum, timing, and location of accelerated electrons near the Sun. Flare-accelerated electrons escaping the Sun can then be tracked into the inner heliosphere through their type- III radio emission observed by RPW (the Radio and Plasma Waves instrument), and by their in situ detection by the Energetic Particle Detector (EPD) suite. In this way, STIX, together with RPW and STEIN, provides direct tracing of the magnetic structure, field line length, and connectivity and is able to magnetically link the heliospheric location observed in situ back to regions at the Sun where the electrons are accelerated. STIX thus plays a key role in connecting the Solar Orbiter in situ and remote sensing observations.

2 Measurement principle

Observationally, STIX determines the location, intensity, spectrum and timing of transient X- ray emission on the Sun at energy ranges that encompass bremsstrahlung emission from both hot thermal plasmas and from energetic electrons. The properties of the electrons that generated the X-rays can be inferred from their X-ray spectrum. The distinction between a thermal plasma and non-thermal electron population is based on the shape of the X-ray spectrum with the latter having a characteristic power law (or broken power law) profile and the former providing a black body spectrum (corresponding to 106 to 108 K). The spectra are very steep and so good spectral resolution is required for their interpretation. There is also an Iron line complex at 6.7 keV which, if isolated, can be interpreted in terms of the thermal electron population. Since a typical flare typically generates both thermal and nonthermal emission, which often are not co-located (for example with locations at the top and footpoints of magnetic loops respectively), both good spatial and good spectral resolution are required. The observational objectives are achieved by imaging the Sun as a function of time and energy with enough spatial, spectral and temporal resolution to match the sources of interest. Comparing the resulting images at different energies yields the X-ray spectra of individual features (e.g. footpoints or flaring loops). Comparing the images as a function of time reveals the temporal behavior of the hot plasma and accelerated electrons. The data can also be combined to yield spatially-integrated light curves and spectra. In all cases, the basic observational datum is a single, photometrically-accurate image corresponding to a well-defined time and energy interval. Within Solar Orbiter constraints, focusing optics is not a feasible option for arcsecond-class hard X-ray imaging. As a result STIX uses an indirect Fourier imaging technique based on X- ray collimation. This is implemented through three mechanically separate modules:: X-ray transparent windows; a passive imager containing front and rear grids; and a Detector/Electronics Module (DEM) containing electronics and passively-cooled X-ray detectors. The Imager is comprised of 32 subcollimators, each of which consists of a pair of well- separated X-ray opaque grids located in front of a corresponding CdTe X-ray detector in the DEM. The X-ray transmission of each grid pair forms a large-scale Moire pattern on the detector. The properties of these Moire patterns are very sensitive to the angular distribution of the incident of the X-ray flux. Although individual CdTe detector pixels associated with each subcollimator provide only ~2 mm spatial resolution, this is sufficient to characterize the Moire

pattern formed by its grids. As a result, high-angular resolution X-ray imaging information is encoded into a set of large scale spatial distributions of counts in the detectors. These distributions can be subsequently decoded on the ground to reconstruct an image of the X- ray source. For each detected X-ray, the detectors provide an output pulse proportional to its energy. By reconstructing images using counts within specific energy intervals, the combined system functions as a high-resolution X-ray imaging spectrometer. Relative pointing information is provided by the spacecraft aspect system while an internal STIX aspect system intermittently establishes the pointing offset of the Instrument Line of Sight (ILS) and the instrument Optical Axis (used for absolute location of images) relative to the spacecraft aspect.