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

• Main Authors: Abarr, Q., Awaki, H., Baring, M. G., Bose, R., De Geronimo, G., Dowkontt, P., Errando, M., Guarino, V., Hattori, K., Hayashida, K., Imazato, F., Ishida, M., Iyer, N. K., Kislat, F., Kiss, M., Kitaguchi, T., Krawczynski, H., Lisalda, L., Matake, H., Maeda, Y., Matsumoto, H., Mineta, T., Miyazawa, T., Mizuno, T., Okajima, T., Pearce, M., Rauch, B. F., Ryde, F., Shreves, C., Spooner, S., Stana, T. -A., Takahashi, H., Takeo, M., Tamagawa, T., Tamura, K., Tsunemi, H., Uchida, N., Uchida, Y., West, A. T., Wulf, E. A., Yamamoto, R.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: arXiv 2020
• Subjects: Instrumentation and Methods for Astrophysics astro-ph.IM; High Energy Astrophysical Phenomena astro-ph.HE; FOS Physical sciences; Antarc*; Antarctica
• Online Access: https://dx.doi.org/10.48550/arxiv.2010.10608; https://arxiv.org/abs/2010.10608

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. : 19 pages, 17 figures. Accepted for publication in Astroparticle Physics