First flight of the X-ray polarimeter X-Calibur

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 InFOCμS grazing incidence X-ray telescope with the goal of observing astrophysical sources. 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. A 1-day test flight of the instrument was performed from Ft.Sumner, NM, in fall 2014. The sensitivity, performance and first results form the flight will be presented.