Concept of a CubeSat-based hard x-ray imaging polarimeter: Cipher

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.