The Next-Generation Hard X-ray Polarimeter XL-Calibur

The polarization of X-rays carries geometrical information about the innermost regions of compact astrophysical objects such as accreting black holes and neutron stars, and magnetars. Different energy ranges can probe various spectral components with different physical origins. For example, in accreting black hole systems, the emission below 10keV is dominated by thermal emission from the accretion disk, whereas the hard X-ray emission above 15keV is dominated by the corona. The balloon-borne scattering polarimeter X-Calibur has obtained the first constraints on the polarization of an accreting neutron star, GX 301-2, in the hard X-ray band between 15 and 30keV. XL-Calibur is the next generation of X-Calibur with significantly increased sensitivity. It utilizes the FFAST hard X-ray optics, which are identical to those of the HXT on the Japanese Hitomi satellite. This X-ray mirror provides a 3-10x larger effective area. In addition, XL-Calibur will use thinner CZT detectors and improved shielding in order to reduce the background. Combined, this will result in an improvement of sensitivity of one order of magnitude. XL-Calibur is scheduled to be launched from Kiruna, Sweden, in spring 2022 and in December 2023 from McMurdo, Antarctica. During these flights, XL-Calibur will observe a variety sources such as the accreting pulsars GX 301-2 and Her X-1, the accreting stellar mass black hole Cyg X-1, the Crab nebula, and additional sources to be chosen based on flux levels at the time of flight. Observations will in part be coordinated with NASA’s recently launched Imaging X-ray Polarimetry Explorer satellite, which is sensitive in the 2-8keV energy range. Here we present the XL-Calibur balloon payload, and give an overview of the measurements enabled by this next-generation instrument.