This paper introduces a second-generation balloon-borne hard X-ray polarimetry mission, XL-Calibur. X-ray polarimetry promises to give qualitatively new information about high-energy astrophysical sources, such as pulsars and binary black hole systems. The XL-Calibur contains a grazing incidence X-ray telescope with a focal plane detector unit that is sensitive to linear polarization. The telescope is very similar in design to the ASTRO-H HXT telescopes that has the world’s largest effective area above ~10 keV. The detector unit combines a low atomic number Compton scatterer with a CdZnTe detector assembly to measure the polarization making use of the fact that polarized photons Compton scatter preferentially perpendicular to the electric field orientation. It also contains a CdZnTe imager at the bottom. The detector assembly is surrounded by the improved anti-coincidence shielding, giving a better sensitivity. The pointing system with arcsecond accuracy will be achieved.
XL-Calibur is a balloon-borne hard X-ray polarimetry mission, the first flight of which is currently foreseen for 2021. XL-Calibur carries an X-ray telescope consists of consists of 213 Wolter I grazing-incidence mirrors which are nested in a coaxial and cofocal configuration. The optics design is nearly identical to the Hard X-ray Telescope (HXT) on board the ASTRO-H satellite. The telescope was originally fabricated for the Formation Flying Astronomical Survey Telescope (FFAST) project. However, the telescope can be used for XL-Calibur, since the FFAST project was terminated before completion. The mirror surfaces are coated with Pt/C depth-graded multilayers to reflect hard X-rays above 10 keV by Bragg reflection. The effective area of the telescope is larger than 300 cm^2 at 30 keV. The mirrors are supported by alignment bars in the housing, and each of the bars has a series of 213 grooves to hold the mirrors. To obtain the best focus of the optics, the positions of the mirrors have to be adjusted by tuning the positions of the alignment bars. The tuning of the mirror positions is conducted using the X-ray beam at the synchrotron facility SPring-8 BL20B2, and this process is called optical tuning. First the positions of the second reflectors are tuned, and then those of the first reflectors are tuned. We did the first optical tuning in Jan 2020. The second tuning will be planned between April to July, 2020. This paper reports the current status of the hard X-ray telescope for XL-Calibur.
X-ray polarimetry promises exciting insights into the physics of compact astrophysical objects by providing two observables: the polarization fraction and angle as function of energy. X-Calibur is a balloon-borne hard x-ray scattering polarimeter for the 15- to 60-keV energy range. After the successful test flight in September 2016, the instrument is now being prepared for a long-duration balloon (LDB) flight in December 2018 through January 2019. During the LDB flight, X-Calibur will make detailed measurements of the polarization of Vela X-1 and constrain the polarization of a sample of between 4 and 9 additional sources. We describe the upgraded polarimeter design, including the use of a beryllium scattering element, lower-noise front-end electronics, and an improved fully active CsI(Na) anticoincidence shield, which will significantly increase the instrument sensitivity. We present estimates of the improved polarimeter performance based on simulations and laboratory measurements. We present some of the results from the 2016 flight and show that we solved several problems, which led to a reduced sensitivity during the 2016 flight. We end with a description of the planned Vela X-1 observations, including a Swift/BAT-guided observation strategy.