Abstract
The hard x-ray imaging spectroscopy system of “Hitomi” x-ray observatory is composed of two sets of hard x-ray imagers (HXI) coupled with hard x-ray telescopes (HXT). With a 12-m focal length, the system provides fine (1 ′ . 7 half-power diameter) imaging spectroscopy covering about 5 to 80 keV. The HXI sensor consists of a camera, which is composed of four layers of Si and one layer of CdTe semiconductor imagers, and an active shield composed of nine Bi4Ge3O12 scintillators to provide low background. The two HXIs started observation on March 8 and 14, 2016 and were operational until 26 March. Using a Crab observation, 5 to 80 keV energy coverage and good detection efficiency were confirmed. The detector background level of 1 to 3 × 10 − 4 counts s − 1 keV − 1 cm − 2 (in detector geometrical area) at 5 to 80 keV was achieved, by cutting the high-background time-intervals, adopting sophisticated energy-dependent imager layer selection, and baffling of the cosmic x-ray background and active-shielding. This level is among the lowest of detectors working in this energy band. By comparing the effective area and the background, it was shown that the HXI had a sensitivity that is same to that of NuSTAR for point sources and 3 to 4 times better for largely extended diffuse sources.
Highlights
The x-ray observatory “Hitomi” was a mission led by JAXA, in collaboration with NASA, ESA, SRON, Canadian Space Agency (CSA), and many other research institutes and universities.[1,2] It was designed to carry out high-resolution x-ray spectroscopy with nondispersive spectrometer in 0.3 to 12 keV, as well as wideband high-sensitivity spectroscopy with imaging up to 80 keV and without imaging up to 600 keV
The hard x-ray imaging spectroscopy is provided by two hard x-ray imagers (HXI), which is the main scope of this paper, coupled with two hard x-ray telescopes (HXT).[3]
HXI-S consists of the HXI camera part and the active shield part. The former includes four layers of double-sided Si strip detectors (DSSD) and a CdTe-DSD imagers, whereas the latter is composed of nine active shield units made of large Bi4Ge3O12 (BGO) scintillator crystals individually coupled to avalanche photo diodes (APD),[13] and the housing structure made of carbon-fiber-reinforced plastic (CFRP) to mechanically hold them
Summary
The x-ray observatory “Hitomi” was a mission led by JAXA, in collaboration with NASA, ESA, SRON, Canadian Space Agency (CSA), and many other research institutes and universities.[1,2] It was designed to carry out high-resolution x-ray spectroscopy with nondispersive spectrometer in 0.3 to 12 keV, as well as. Wideband high-sensitivity spectroscopy with imaging up to 80 keV and without imaging up to 600 keV The former can diagnose the temperature, ionization state, and dynamical motion of celestial plasma, and the latter can distinguish hotthermal and nonthermal emission from the main thermal component. The HXI adopts the low background technology verified by the hard x-ray detector (HXD)[8,9] onboard the “Suzaku” satellite, in particular the well-shaped-thick active-shield to realize low background.[10] The HXT employs a newly developed multilayer super mirror optics It accumulates hard x-ray photons into the low background imager, and improves the sensitivity by ∼100 times compared with its predecessor, the HXD. Detail of the in-orbit performance and instrument response is reported in Hagino et al.[11]
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