XL-CALIBUR

XL-Calibur is a collaboration amongst scientists from the United States, Japan, and Sweden, led by the Principal Investigator, Henric Krawczynski from Washington University (St. Louis). The mirror uses a flight spare from the Hitomi mission, a gondola and pointing control mechanism developed at NASA’s Wallops Space Flight Center, and a fully active shield developed at KTH in Sweden. The mission is flown on Long Duration Balloon (LDB) flights launched from McMurdo (Antarctic) or Jukkasjärvi (Sweden) as part of NASA’s Scientific Ballooning program.

The main objectives of XL-Calibur are to measure polarization at hard X-ray wavelengths for a selection of bright sources. These measurements address the following science goals:

• constrain the shape and physical properties of the X-ray bright accretion disk coronas of mass accreting stellar mass black holes and neutron stars;
• use polarization measurements of the X-rays from strongly magnetized neutron stars to test predictions of strong-field Quantum electrodynamics;
• identify the origin of the X-rays from rotation powered neutron stars like the Crab Pulsar and Nebula and the accretion powered GX301-2 X-ray pulsar;

XL-Calibur is mounted on a gondola carried by an extreme altitude balloon into the stratosphere at roughly 40 km (130,000 feet) float altitude. At this altitude, the Earth’s atmosphere is transparent to X-rays above 15 keV. Special coatings on the X-ray mirror allow XL-Calibur to focus X-rays in the 15 keV to 80 keV energy range.

Currently two flights are planned: one from Kiruna (Sweden) scrutinizing Northern Hemisphere sources, and one from the McMurdo Station (Antarctica) circling the Antarctic observing sources in the Souther Hemisphere. The flight from Sweden to Canada will allow for 6-7 days of observations. The flight circling the Antarctic continent will allow for 8-55 days of observations. The XL-Calibur observations will be coordinated with complimentary observations with NASA’s Imaging X-ray Polarimetry Explorer (IXPE), the The Neutron Star Interior Composition Explorer Mission (NICER), and with the Nuclear Spectroscopic Telescope Array (NuSTAR) mission.

The XL-Calibur data will be delivered and hosted by the HEASARC.