X-ray polarimetry of compact objects with XL-Calibur

XL-Calibur is a balloon-borne hard X-ray polarimetry mission, which will study compact objects, such as pulsars and mass-accreting neutron stars/black holes, in a new way.

The aim is to advance understanding of high-energy emission mechanisms and source geometry complementary to imaging, spectroscopy, and timing. Spectropolarimetric observations (15-80 keV) will be conducted from a stratospheric balloon-borne platform. XL-Calibur significantly improves on previous missions (e.g.

PoGO+, X-Calibur) with a several-fold increase in signal rate together with an order-of-magnitude improvement in the signal-to-background ratio.

Thanks to previous SNSA funding, KTH has assumed a central role in the development of XL-Calibur: (i) development and operation of the anticoincidence shield, which encapsulates the polarimeter to mitigate measurement backgrounds, (ii) participation in mission integration and testing activities, (iii) data reduction, analysis and simulation work required to provide technical and scientific results.

XL-Calibur was launched from Esrange on 12th July 2022 for a 7-day long flight to Northern Canada, with the Crab pulsar and black-hole binary Cyg X-1 as primary targets. Regrettably, technical problems precluded science results. The next XL-Calibur launch will also be from Esrange.

XL-Calibur has been approved for launch, which is currently expected to be possible no earlier than the end of May when stable westward-flowing winds are predicted at 5 hPa (~40 km) altitude.

In addition to the Esrange 2024 flight, NASA Astrophysics Research and Analysis (APRA) funding includes an XL-Calibur flight from McMurdo base on Antarctica. The earliest feasible launch date is late 2026/early 2027.

A McMurdo flight will extend the XL-Calibur science programme to Southern sky sources, such as the accreting neutron stars GX301-2 and Vela X-1, as well as several bright transient sources (>200 mCrab).