XL-Calibur -- a second-generation balloon-borne hard X-ray polarimetry mission

TL;DR

XL-Calibur is an advanced balloon-borne X-ray polarimetry mission designed to improve sensitivity and precision in measuring X-ray polarization from astrophysical sources, building on previous missions with enhanced instrumentation.

Contribution

It introduces a second-generation design with a larger mirror, thinner CZT detectors, and improved shielding, enabling more sensitive X-ray polarization measurements from balloon platforms.

Findings

Achieves ~2% minimum detectable polarization for 1 Crab source in 1 day

Energy resolution of ~5.9 keV at 40 keV

Design improvements lead to substantially better sensitivity than previous missions

Abstract

XL-Calibur is a hard X-ray (15-80 keV) polarimetry mission operating from a stabilised balloon-borne platform in the stratosphere. It builds on heritage from the X-Calibur mission, which observed the accreting neutron star GX 301-2 from Antarctica, between December 29th 2018 and January 1st 2019. The XL-Calibur design incorporates an X-ray mirror, which focusses X-rays onto a polarimeter comprising a beryllium rod surrounded by Cadmium Zinc Telluride (CZT) detectors. The polarimeter is housed in an anticoincidence shield to mitigate background from particles present in the stratosphere. The mirror and polarimeter-shield assembly are mounted at opposite ends of a 12 m long lightweight truss, which is pointed with arcsecond precision by WASP - the Wallops Arc Second Pointer. The XL-Calibur mission will achieve a substantially improved sensitivity over X-Calibur by using a larger effective area X-ray mirror, reducing background through thinner CZT detectors, and improved anticoincidence shielding. When observing a 1 Crab source for $t_{\rm day}$ days, the Minimum Detectable Polarisation (at 99% confidence level) is $\sim$2$\%\cdot t_{\rm day}^{-1/2}$. The energy resolution at 40 keV is $\sim$5.9 keV. The aim of this paper is to describe the design and performance of the XL-Calibur mission, as well as the foreseen science programme.