We propose a novel concept of a ultracompact hard X-ray imaging polarimeter system that utilizes the combination of a fine-pixel CMOS imaging sensor and a narrow field-of-view coded aperture mask with multiple different random patterns. An instrument using this concept can be installed in the cost-effective 6U CubeSat mission cipher, providing us a quick opportunity to demonstrate potentials of the semiconductor photoabsorption-type polarimeter by realizing imaging polarimetry of the brightest objects in an energy band of 10–30 keV. Polarization of this energy band, which is the lower part of hard X-rays, has been unexplored in spite of its great scientific importance. The science targets of cipher include particle acceleration at relativistic shocks in the Crab Nebula, the accretion flow geometry in Cygnus X-1, and the anisotropy of electrons accelerated by bright solar flares. We show by proof-of-concept experiments in SPring-8 that a CMOS sensor with a pixel size of 2.5 µm has polarization sensitivity at energies of 10, 16, and 24 keV with modulation factors of 4.24% ±0.03%, 11.82%±0.06%, and 15.15%±0.25%, respectively. We also demonstrate that the coded aperture imaging with the different random patterns achieves artifact-reduced image decoding with an angular resolution of 30 arcseconds. The combination of these methods can be naturally extended to imaging polarimetry with high energy and angular resolutions.
Understanding and reducing in-orbit instrumental backgrounds are essential to achieving high sensitivity in hard x-ray astronomical observations. The observational data of the Hard X-ray Imager (HXI) onboard the Hitomi satellite provide useful information on the background components due to its multilayer configuration with different atomic numbers: the HXI consists of a stack of four layers of Si (Z = 14) detectors and one layer of cadmium telluride (CdTe) (Z = 48, 52) detector surrounded by well-type Bi4Ge3O12 active shields. Based on the observational data, the backgrounds of the top Si layer, the three underlying Si layers, and the CdTe layer are inferred to be dominated by different components, namely, low-energy electrons, albedo neutrons, and proton-induced radioactivation, respectively. Monte Carlo simulations of the in-orbit background of the HXI reproduce the observed background spectrum of each layer well, thereby quantitatively verifying the above hypothesis. In addition, we suggest the inclusion of an electron shield to reduce the background.
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