Concept of the cipher Mission: a CubeSat for the Imaging Polarimetry in the Hard X-ray Band

The polarization information of non-thermal X-rays is important for the high energy astrophysics to reveal the geometrical or magnetic field structure of the emission environment. The IXPE satellite (Imaging X-ray Polarimetry Explorer) will be the world-first polarimeter which has an ability of imaging in the soft X-ray band. On the other hand, in the hard X-ray band where non-thermal X-rays dominate, there is no existing observatory with the imaging capability (e.g., PoGO+, Hitomi-SGD). Our mission named cipher (coded imaging polarimeter of high energy radiation) aims for this unexplored field by the combination of a CMOS imaging sensor with fine pitch pixels and multiple coded aperture masks with different patterns. A small detector pixel size allows us to track the photoelectron, which gives us a direct information on the original polarization angle of incident photons. A coded aperture with a small aperture size can achieve a fine angular resolution of ~30 arcsec (FWHM), even onboard a compact observatory such as a CubeSat. Focusing on the energy band from 10 to 30 keV and the target source to bright ones, our cipher mission will achieve scientifically sufficient results even with the limited effective area and the short life time assuming a CubeSat: e.g., the first imaging polarimetry of the Crab Nebula in the hard X-ray band. In addition to introduce our detailed concept, we report on the experimental demonstration using a synchrotron photon beam in SPring-8 in Japan.