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

• Published in: Astroparticle physics Vol. 126; p. 102529
• 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: Journal Article
• Language: English
• Published: Goddard Space Flight Center Elsevier B.V 01.03.2021; Elsevier
• Subjects: Compact objects; Optics; Scientific ballooning; X-ray polarimetry; Compact objects; Scientific ballooning; X-ray polarimetry
• ISSN: 0927-6505; 1873-2852; 1873-2852
• DOI: 10.1016/j.astropartphys.2020.102529

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 tdaydays, the Minimum Detectable Polarisation (at 99% confidence level) is ∼2%·tday−1/2. The energy resolution at 40 keV is ∼5.9 keV. The aim of this paper is to describe the design and performance of the XL-Caliburmission, as well as the foreseen science programme.