Simulating and Testing the Polarimetric Response of the X-ray Polarimetry Telescope XL-Calibur

Abstract: X-ray polarimetry, the study of the polarisation of X-ray light, is a powerful and rapidly developing tool for astrophysics, which promises to help answer outstanding questions about the physics of extreme objects such as pulsars, X-ray binaries and active galactic nuclei. The balloon-borne telescope XL-Calibur will be the first instrument to study the polarisation of hard X-rays (with energies between 10 and 100 keV) in detail, correlating with the soft X-ray observations of IXPE to provide further tests of polarisation-dependent x-ray emission models in extreme objects. The working principles of XL-Calibur are described, together with the necessary steps to measure the polarisation of X-rays. In these steps, Geant4 simulations of the telescope play a vital role, and the simulations are thus described in detail, together with the experiments done to validate the simulations. These experiments were performed at Esrange, Sweden during the XL-Calibur flight campaign in May of 2022, and the experimental setup and design of the validation experiments are described, as well as the specific simulations performed to replicate the experiment. The simulations show very good agreement with validation experiments, achieving a simulated modulation factor (a measure of the polarimetric response intrinsic to the detector) of 41.88% ± 0.17%, within one standard deviation of the measured 41.95% ± 0.18%. The optical effects of the XL-Calibur X-ray mirror is also simulated to good agreement with experimental results, necessary for simulating flight observations. Thus, the simulations can be used to simulate XL-Calibur for polarisation measurements and analysis. Furthermore, the effect on polarisation parameters of the mirror focal spot being offset is investigated. It is shown that it affects the modulation factor and thus the measured polarisation parameters, increasing the importance of using simulations to replicate and compensate for these effects during a data-taking flight with XL-Calibur.