X-ray polarimetry promises to give qualitatively new information about high-energy astrophysical sources, such as binary black hole systems, micro-quasars, active galactic nuclei, and gamma-ray bursts. We designed, built and tested a hard X-ray polarimeter, X-Calibur, to be used in the focal plane of the balloon-borne InFOCuS grazing incidence hard X-ray telescope. X-Calibur combines a low-Z Compton scatterer with a CZT detector assembly to measure the polarization of 20-60 keV X-rays making use of the fact that polarized photons Compton scatter preferentially perpendicular to the electric field orientation. The X-Calibur detector assembly is completed, tested, and calibrated; a first flight is scheduled from Ft. Sumner, NM, in fall 2014. In principal, a similar space-borne experiment could be operated in the 5–100 keV regime. X-Calibur achieves a high detection efficiency of order unity.
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