Presentation #314.01 in the session X-ray and Radio Facilities and Instruments.
X-ray polarimetry is a new and relatively unexplored window into the high-energy universe. The polarization properties of observed photons carry information about the inner geometries of the compact objects in X-ray binaries. X-ray polarimetry can thus help to separate contributions from the accretion disk and coronal emission of black holes. It can also help to distinguish competing models of emission from neutron stars and magnetars, and help identify the region within the Crab pulsar’s magnetosphere that emits high-energy photons. XL-Calibur is a second-generation balloon-borne scattering polarimeter that will measure in the hard X-ray (15-80keV) regime. Incident photons are focused by an X-ray mirror, scatter off of a central Beryllium rod, and are then detected by pixelated Cadmium Zinc Telluride (CZT) detectors that surround it. Photons scatter preferentially perpendicular to their polarization direction. They have a high probability of scattering, with ~85% of 30keV X-rays scattering from the Be rod to one of the CZT detectors. X-Calibur was the first iteration of this mission, which measured the X-ray polarization of GX 301-2 between December 29, 2018 and January 1, 2019. This was the first constraint on polarization of this pulsar. XL-Calibur will have much lower background rates (<4%) and a significant better sensitivity (~1 order of magnitude) than X-Calibur. XL-Calibur is expected to be launched from Esrange, Sweden in May 2022 with planned targets of the Crab Nebula, Cyngus X-1, and a third target of interest. A second flight is planned for Antarctica in late 2023. XL-Calibur’s results will complement the results from NASA’s IXPE mission, which is currently measuring the polarization of X-rays in the 2-10keV energy range. This talk will discuss an overview of the XL-Calibur mission, review the scientific potential of its X-ray polarimetry measurements, and discuss future plans for the mission.