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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.

X-ray polarimetry holds the promise to resolve the inner regions of compact systems like mass accreting black holes in X-ray binaries and X-ray bright neutron stars. For example, spectropolarimetric observations of pulsars and pulsar wind nebulae can constrain the geometry and locale of particle acceleration in these sources. We designed and built X-Calibur, a hard x-ray polarimeter which was flown in the focal plane of the InFOCµS grazing incidence mirror in the fall of 2014 from Fort Sumner (NM). Additional flights are planned for the fall of 2016 from Fort Sumner, as well as for the 2018/19 Antarctic season from McMurdo (Antarctica). 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 scattering slab with high-Z Cadmium Zinc Telluride detectors to photo-absorb the scattered X-rays, X-Calibur achieves a high detection efficiency of almost unity and a large modulation factor. 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, results of laboratory calibration measurements characterizing the performance of the instrument, as well as background measurements taken during the first flight.

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.