National Aeronautics and Space Administration

XL-Calibur

The XL-Calibur mission is a balloon-borne X-ray telescope set to flown in May 2022 from Kiruna, Sweden, and again from McMurdo Station in Antartica in 2023. The flight durations are anticipated 5–7 days from Kiruna and 5–55 days for second McMurdo flight. The balloon circles the polar region at stratospheric attitudes (roughly 39.6 km; 130,000 feet) where atmospheric attenuation of X-rays is reasonably slight.
A proof-of-concept mission, X-Calibur, was previously flown in the Earth’s stratosphere from McMurdo Station in Antarctica (29 Dec 2018 - 1 Jan 2019). X-Calibur was also flown in a much shorter test flight in September 2016 from Fort Sumner, New Mexico; no astrophysical data was collected, but the instruments did characterized the high-altitude background noise environment.

XL-Calibur uses a flight spare X-ray telescope from the Hitomi mission with a 12 m focal length, mounted in a carbon fibre tube and aluminium optical bench truss: the X-Calibur Dec 2018 flight used the smaller INFOCμS mirror, which also had a shorter 8 m focal length. The detector is sensitive to the linear polarization of the incident hard X-rays. The instrument is shielded from atmospheric gamma- and cosmic rays with anti-coincidence shielding. The system is deployed in a gondola beneath a balloon and at the end the mission, the apparatus is parachuted back to the surface and recovered for reflight. The flight program calls for a modest list of proposed targets in the northern (Kiruna) and southern (McMurdo) skies; The December 2018 flight observed GX 301-2 and Vela X-1, but was cut short late in on Jan 1 2019 when the balloon developed a leak.

Mission Characteristics

Lifetime

Energy Range

Special Features

Payload

• Polarimeter. The detector is a single beryllium rod, surrounded by Cadmium Zinc Telluride (CZT) detectors (2mm thick CZT in X-Calibur; 0.8 mm thick in XL-Calibur). A CZT detector at the rear of the scattering rod is used to monitor the position of the source within the field of view. The detector is also equipped with Bismuth Germanium Oxide (BGO) anti-coincidence detectors to shield the instrument to background radiation.
  
  • Energy range : 15–60 keV
    
  • Effective area : 180 cm² at 30 keV (XL-Calibur)
    
  • Focal length : 8 meter (X-Calibur); 12 meter (XL-Calibur)
  • Angular resolution : ∼2 arcmin
    
  • Timing resolution: ∼1 μs
    
  • Energy Resolution: < 7 eV at 6 keV

• Mirror Assembly and Support Truss. For XL-Calibur, the mirror is a flight spare from the Hitomi Hard X-Ray Telescope, and originally made for the FFAST mission. The mirror and detector assembly are mounted at opposite ends of a truss (12 m for XL-Calibur; 8 m long in X-Calibur) made of aluminium-carbon fibre
• Wallops Arc Second Pointer (WASP). The truss system with mirror and detector is mounted on a gondola equipped with the WASP pointing system. The system is designed to achieve absolute pointing knowledge to 15 arcsec accuracy and pointing precision of less than 1 arcsec.

Science Goals

• Observation of hard X-ary emission from stellar-mass black hole in X-ray binaries and constrain the property of X-ray bright coronas.
• Distinguish between competing emission models of the rotation-powered Crab pulsar and cleanly separate contributions from the pulsar and from the nebula.
• Measurements of highly-magnetised accreting pulsars at the energies of their Cyclotron Resonant Scattering Features, where the competition of QED and plasma birefringence is expected to give high and strongly energy-dependent polarisation

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