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

TL;DR

The X-Calibur balloon-borne telescope successfully measured the polarization of high-energy X-rays during its 2018/19 flight, providing valuable data and insights for future X-ray polarimetry missions.

Contribution

This paper reports the first constraints on hard X-ray polarization of an accretion-powered pulsar from a balloon flight and details the instrument's performance and calibration.

Findings

Successful polarization measurement during flight

Identification of higher-than-expected energy threshold issues

Performance insights for future missions like XL-Calibur

Abstract

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