In-orbit performance and calibration of the Hard X-ray Imager onboard Hitomi (ASTRO-H)

The Hard X-rayImager (HXI) onboard Hitomi (ASTRO-H) is an imaging spectrometercovering hard x-rayenergies of 5 to 80 keV. Combined with the Hard X-rayTelescope, it enables imaging spectroscopywith an angular resolutionof 1′.7 half-power diameter, in a fieldof viewof 9′  ×  9′. The main imager is composed of four layers of Si detectors and one layer of CdTe detector, stacked to cover a wide energy band up to 80 keV, surrounded by an active shield made of Bi4Ge3O12 scintillator to reduce the background. The HXI started observations 12 days before the Hitomi loss and successfully obtained data from G21.5–0.9, Crab, and blank sky. Utilizing these data, we calibrate the detector response and study properties of in-orbit background. The observed Crab spectra agree well with a powerlaw model convolved with the detector response, within 5% accuracy. We find that albedo electrons in specified orbit strongly affect the background of the Si top layer and establish a screening method to reduce it. The background level over the full fieldof viewafter all the processing and screening is as low as the preflight requirement of 1  −  3  ×  10−4  counts s−1 cm−2 keV−1.