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.
The coded aperture imaging technique is a useful method of x-ray imaging in observational astrophysics. However, the presence of imaging noise or so-called artifacts in a decoded image is a drawback of this method. We propose a coded aperture imaging method using multiple different random patterns for significantly reducing the image artifacts. This aperture mask contains multiple different patterns each of which generates a different artifact distribution in its decoded image. By summing all decoded images of the different patterns, the artifact distributions are cancelled out, and we obtain a remarkably accurate image. We demonstrate this concept with imaging experiments of a monochromatic 16-keV hard x-ray beam at the synchrotron photon facility SPring-8, using the combination of a complementary metal-oxide-semiconductor image sensor and an aperture mask that has four different random patterns composed of holes with a diameter of 27 μm and a separation of 39 μm. The entire imaging system is installed in a 25-cm-long compact size and achieves an angular resolution of <30 arc sec (full-width at half-maximum). In addition, we show by Monte Carlo simulation that the artifacts can be reduced more effectively if the number of different patterns increases to 8 or 16.
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