The x-ray polarization of compact objects in x-ray binaries allows us to understand the complex spacetimes surrounding these sources. XL-Calibur is a state-of-the-art, balloon-borne telescope that measures the linear polarization of stellar-mass black holes, neutron stars, and nebulae in the 15-80 keV energy band. The selected energy range allows for observing coronal emission from black holes while also enabling us to narrow down on emission models from neutron stars, pulsars, and magnetars. Early in 2024, XL-Calibur will be launched from Kiruna, Sweden for approximately 10 days to observe Cyg X-1 and Cyg X-3, or other sources chosen based on flux levels at the time of flight. Observations might be coordinated with the recently launched Imaging x-ray Polarimetry Explorer mission which measures polarization in the complimentary 2-8 keV band. Combined XL-Calibur and IXPE observations will yield information on both soft and hard x-rays allowing us to decompose the total emission from black holes into thermal disk and coronal. We discuss the characterization of the XL-Calibur CdZnTe detectors, the telescope mirror and truss setup, and preliminary results from our most recent flight.
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.
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