The hard x-ray polarimeter X-Calibur

X-ray polarimetry promises unique information about astrophysical objects such as binary black hole systems, neutron stars, microquasars, active galactic nuclei, and gamma-ray bursts. We designed and built X-Calibur, a hard x-ray polarimeter to be flown in the focal plane of the InFOCS grazing incidence mirror in the fall of 2014 from Fort Sumner (NM)¹. During the one-day flight X-Calibur will measure spectrum and polarization of x-rays in the 20-80keV range from up to five selected sources. X-Calibur exploits the fact that polarized photons scatter preferentially in a direction orthogonal to the orientation of their electric field vector. By combining a low-Z scatterer with high-Z Cadmium Zinc Telluride (CZT) detectors to photo-absorb the scattered x-rays, X-Calibur achieves a high detection efficiency of almost unity and a large modulation factor close to the theoretical limit given by the physics of Compton scattering. We have calibrated and tested X-Calibur extensively in the laboratory at Washington University and at the Cornell High-Energy Synchrotron Source (CHESS). Measurements using the highly polarized synchrotron beam at CHESS confirm the polarization sensitivity of the instrument. In this paper we report on the design of X-Calibur and results of laboratory calibration measurements characterizing the performance of the instrument.