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

TL;DR

This paper reports on the in-orbit performance, calibration, and background analysis of the Hitomi Hard X-ray Imager, demonstrating its effective imaging spectroscopy capabilities and background reduction techniques in space.

Contribution

It provides the first in-orbit calibration and background characterization of the Hitomi HXI, including a screening method to mitigate background from albedo electrons.

Findings

Crab spectra match the powerlaw model within 5% accuracy.

Background levels meet pre-flight requirements after screening.

Albedo electrons significantly influence Si detector background.

Abstract

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 $9^\prime\times9^\prime$. 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 $1$-$3\times10^{-4}$ counts s$^{-1}$ cm$^{-2}$ keV$^{-1}$.