A Bragg reflection x-ray polarimeter using curved crystals

Combining high-resolution spectroscopy with polarization measurements provides a powerful method for investigating astrophysical phenomena that cannot be spatially resolved. In the x-ray band in particular, analyzing the polarization structure of broadened iron emission lines originating near black holes can reveal the geometry of the inner accretion disk and corona. Furthermore, fluorescent x-rays emitted when high-energy electrons propagate through plasma and excite ions can exhibit polarized fine structures, offering insights into extreme physical processes such as particle acceleration and magnetic reconnection. To realize such observational capabilities, we are developing a novel Bragg reflection x-ray optics system with rotating paraboloidal geometry. This design is based on the theoretical property that Bragg reflection achieves a modulation factor of 100% at an incident angle of 45 degrees. By employing bent crystals, the reflection—typically monochromatic in flat crystals—can be extended to a broader energy range, as different positions reflect different photon energies. Additionally, by placing the detector intentionally out of focus, the reflected x-rays can be spatially dispersed, enabling high-resolution spectroscopy. It has also been experimentally confirmed that the integrated reflectivity of bent crystals exceeds that of flat crystals, similar to the behavior of mosaic crystals. This new system, named ParaDAXAS, enables simultaneous measurement of polarization degree and polarization angle with high spectral resolution. Such capability, which was previously difficult to achieve, offers a highly promising approach for future x-ray spectropolarimetric missions in astrophysics.