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

Kavli Affiliate: Shinichiro Takeda

| First 5 Authors: Kouichi Hagino, Kazuhiro Nakazawa, Goro Sato, Motohide Kokubun, Teruaki Enoto

| Summary:

The Hard X-ray Imager (HXI) onboard Hitomi (ASTRO-H) is an imaging
spectrometer covering hard X-ray energies of 5-80 keV. Combined with the hard
X-ray telescope, it enables imaging spectroscopy with an angular resolution of
$1^prime.7$ half-power diameter, in a field of view of
$9^primetimes9^prime$. The main imager is composed of 4 layers of Si
detectors and 1 layer of CdTe detector, stacked to cover wide energy band up to
80 keV, surrounded by an active shield made of BGO 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 Si top layer,
and establish a screening method to reduce it. The background level over the
full field of view after all the processing and screening is as low as the
pre-flight requirement of $1$-$3times10^{-4}$ counts s$^{-1}$ cm$^{-2}$
keV$^{-1}$.

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