Abstract
The soft x-ray spectrometer (SXS) instrument was launched aboard the Astro-H (Hitomi) observatory on February 17, 2016. The SXS is based on a high-sensitivity x-ray calorimeter detector system that has been successfully deployed in many ground and suborbital spectrometers. The instrument was to provide essential diagnostics for nearly every class of x-ray emitting objects from the atmosphere of Jupiter to the outskirts of galaxy clusters, without degradation for spatially extended objects. The SXS detector system consisted of a 36-pixel cryogenic microcalorimeter array operated at a heat sink temperature of 50 mK. In preflight testing, the detector system demonstrated a resolving power of better than 1300 at 6 keV with a simultaneous bandpass from below 0.3 keV to above 12 keV with a timing precision better than 100 μs. In addition, a solid-state anticoincidence detector was placed directly behind the detector array for background suppression. The detector error budget included the measured interference from the SXS cooling system and the spacecraft. Additional margin for on-orbit gain stability and on-orbit spacecraft interference were also included predicting an on-orbit performance that meets or exceeds the 7-eV FWHM at 6-keV requirement. The actual on-orbit spectral resolution was better than 5 eV FWHM at 6 keV, easily satisfying the instrument requirement. Here, we discuss the actual on-orbit performance of the SXS detector system and compare this to performance in preflight testing and the on-orbit predictions. We will also discuss the on-orbit gain stability, additional on-orbit interference, and measurements of the on-orbit background.
Highlights
The soft x-ray spectrometer (SXS) instrument[1] is a broadband nondispersive x-ray spectrometer (XRS) that was launched on February 17, 2016, as part of the Astro-H x-ray observatory
For the Perseus cluster observations, the observation itself was used to correct the differential gain by tracking the position of the He-like Fe Kα line as a function of time and extrapolating to the full array fiducial measurement on March 19.3 We note that the Perseus and other observations that were completed before the loss of the Astro-H mission were made before the instrument was fully deployed
The SXS telemeters all cosmic ray events interacting in the anticoincidence detector (ACD) to the ground and the veto timing with the main array occurs in the pipeline analysis software
Summary
The soft x-ray spectrometer (SXS) instrument[1] is a broadband nondispersive x-ray spectrometer (XRS) that was launched on February 17, 2016, as part of the Astro-H x-ray observatory. Very high resolving powers can be achieved, including resolving powers of over 3000 at 6 keV and over a substantial bandpass.[10] The pixels in an array of microcalorimeters are spatially distinct, in that, with a suitable telescope, each pixel is its own spatially independent spectrometer This type of instrument is well suited for observing extended objects, such as galaxy clusters, and supernova remnants without the spatial–spectral confusion associated with slitless dispersive spectrometers, such as those on XMMNewton and Chandra. There is an additional ADR stage that provides redundancy for the liquid helium system and can indefinitely extend SXS operations even after the liquid helium is exhausted.[13] A majority of the SXS FPA design is to isolate the thermally, mechanically, and electrically sensitive detector system from the intricate SXS cooling system, as well as the complex Astro-H spacecraft. Note that details of the pulse-processing algorithm, including the on-orbit performance of event-grading, triggering, and timing performance, are described in detail by Ishisaki et al.[5]
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