X-CALIBUR / InFOCuS

X-Calibur is a sensitive hard X-ray polarimeter built to be installed in the focal plane of a focussing hard X-ray telescope. X-Calibur measures polarization based on detecting the scattering angle of Compton scattered X-ray photons. The photons scatter preferentially into a direction perpendicular to the orientation of the electric field vector. The experiment uses a low-atomic-number (plastic) slab as scatterer and high-atomic-number solid state detector ("CZT" detectors) to absorb the scattered photons. It operates in the 20-80 keV range. The instrument combines a >80% detection efficiency, with a Full Width Half Maximum (FWHM) energy resolution of 5 keV, and a very low background level. X-Calibur is the only hard X-ray polarimeter which achieves a sensitivity close to the limiting sensitivity dictated by the physics of Compton interactions.

X-Calibur is mounted in a presurized vessel at the focal plane of the InFOCuS X-ray telescope, a stabilized balloon-borne platform developed during the 2000's decade at the Goddard Space Flight Center. A Wolter grazing incidence mirror specialy developed for InFOCµS at the Nagoya University in Japan -composed of 255 aluminum shells coated with platinum and carbon layers- focused the incoming X-rays onto the polarimeter. The focal length of the mirror is 8 m and the field of view was FWHM = 10 arcmin. The telescope truss of InFOCµS is only coupled to the gondola by a ball joint in a support cup with floating oil, allowing for full inertial pointing of the telescope with an accuracy of 700 and 1500 RMS in altitude and azimuth, respectively. To maintain the decoupling between truss and gondola, any communication between the two systems is done by wireless connections.

Balloon launched on: 9/24/2014 at 14:16 utc
Launch site: Scientific Flight Balloon Facility, Fort Sumner, (NM), US  
Balloon launched by: Columbia Scientific Balloon Facility (CSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon  
Flight identification number: 653N
End of flight (L for landing time, W for last contact, otherwise termination time): 9/24/2014 at 21:55 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 7 h 40 m
Landing site: 40 miles W of Ft.Sumner, New Mexico, US

During the flight, valuable background data was acquired, however due to a failure of the pointing system, no cosmic X-ray sources were observed.

• Design and Performance of the X-ray polarimeter X-Calibur Journal of Astronomical Instrumentation, Volume 3, Issue 2, id. 1440008
• Modern Assessment of the High-Energy Background Environment at Small Atmospheric Depths Using the X-Calibur X-Ray Polarimeter and Its Implications Ph.D Dissertation by Rashied Baradaran Amini (2016)
• X-Calibur website Washington University, St. Louis