The Soft X-ray Spectrometer (SXS) consists of the Soft X-ray Telescope
(SXT-S), the X-ray Calorimeter Spectrometer (XCS) and the cooling system.
The XCS is a 32 channel system with an energy resolution of 7 eV between 0.3—12 keV. Micromachined, ion-implanted silicon is the basis of the thermistor array, and 8-micron-thick HgTe absorbers provide high quantum efficiency across the 0.3–12~keV band. With a 6-m focal length, the 0.83 mm pixel pitch corresponds to 0.48 arcmin, giving the array a field of view of 2.85 arcmin on a side. In order to obtain high energy resolution, the XCS cooling system must cool the array to 50~mK.
The SXS science objectives require a mirror with larger effective area than those flown on Suzaku, especially in the Fe K band. The SXS effective area at 6 keV is 210 cm2, a 60 % increase over the Suzaku XRS, while at 1~keV the SXS has 160 cm2, a 20 % increase. If we adopt a thin filter, the effective areaat 1 keV increases to +250~cm2. The required angular resolution is 1.7 arcmin, HPD, comparable to the orbit performance of the mirrors on Suzaku.
The broad bandpass of SXS encompasses the critical inner-shell emission and absorption lines of Fe I-XXVI between 6.4 and 9.1 keV. Fe lines are useful because of their (1) strength, due to the high abundance and large fluorescent yield (30 %), (2) spectral isolation from other lines, and (3) relative simplicity of the atomic physics. Fe K emission lines reveal conditions in plasmas with temperatures between 107 and 108 K, typical values for stellar accretion disks, SNRs, clusters of galaxies, and many stellar coronae. In cooler plasmas, Si, S, and Fe fluorescence and recombi-nation occurs when an X-ray source illuminates nearby neutral material. Fe emission lines provide powerful diagnostics of non-equilibrium ioniza-tion due to innershell K-shell transitions from Fe XVII–XXIV.
SXS uniquely performs high-resolution spectroscopy of extended sources. In contrast to a grating, the spectral resolution of the calorimeter is unaffected by source size because it is non-dispersive. SXS makes possible high-resolution spectroscopy of sources inaccessible to current grating instruments.