The X-ray scattering polarimeter X-Calibur

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.