Performance of the X-Calibur Hard X-ray Polarimetry Mission During the 2018-2019 Long Duration Balloon Flight

X-ray polarimetry promises geometrical information about compact astrophysical objects such as accreting black holes and neutron stars that are too small to be imaged at any wavelength. While some information about the source geometry can be derived from spectral and timing measurements of the high-energy emission, results are often model dependent. X-ray polarimetry can break these degeneracies by providing two new observables, namely the Stokes parameters Q and U.X-Calibur is a balloon-borne hard X-ray polarimeter covering the 20-40keV energy range. It consists of an 8m long optical bench carrying the InFOCμS hard X-ray mirror, and a scattering polarimeter at its focal point. The optical bench is pointed with arc-second precision by the Wallops Arc-Second Pointer (WASP). The polarimeter consists of a Beryllium scattering element at the focal point of the mirror surrounded by CZT detectors on 4 sides to measure azimuthal scattering angle and energy of the scattered photons.X-Calibur was flown on a relatively short stratospheric balloon flight from McMurdo (Antarctica) from Dec. 29, 2018 to Jan. 1, 2019, during which it observed GX 301-2 and Vela X-1. Here, we report on the performance of the instrument during this flight and the measured background levels.