Performance of the X-Calibur Hard X-Ray Polarimetry Mission during its\n 2018/19 Long-Duration Balloon Flight

Abstract

X-Calibur is a balloon-borne telescope that measures the polarization of\nhigh-energy X-rays in the 15--50keV energy range. The instrument makes use of\nthe fact that X-rays scatter preferentially perpendicular to the polarization\ndirection. A beryllium scattering element surrounded by pixellated CZT\ndetectors is located at the focal point of the InFOC{\\mu}S hard X-ray mirror.\nThe instrument was launched for a long-duration balloon (LDB) flight from\nMcMurdo (Antarctica) on December 29, 2018, and obtained the first constraints\nof the hard X-ray polarization of an accretion-powered pulsar. Here, we\ndescribe the characterization and calibration of the instrument on the ground\nand its performance during the flight, as well as simulations of particle\nbackgrounds and a comparison to measured rates. The pointing system and\npolarimeter achieved the excellent projected performance. The energy detection\nthreshold for the anticoincidence system was found to be higher than expected\nand it exhibited unanticipated dead time. Both issues will be remedied for\nfuture flights. Overall, the mission performance was nominal, and results will\ninform the design of the follow-up mission XL-Calibur, which is scheduled to be\nlaunched in summer 2022.\n