X-rays from Warped Black Hole Accretion Disks

In this thesis, I present the results from my research to better understand accretion onto black holes and neutron stars based on spectropolarimetric X-ray observations. I have developed a general relativistic ray-tracing code which simulates X-rays from warped accretion disks around black holes. I used this to predict the polarization of the thermal X-ray emission and the energy spectrum the reflected power law emission. Both of these can be used to measure properties of black hole systems, such as the spin parameter and the inclination of the observer to its spin axis. My results enable the measurement of these parameters with improved accuracies and with a different set of systematic errors. The methods discussed can be applied to the data of existing X-ray satellites, such as Chandra and XMM-Newton, and upcoming spaceborne missions, such as the Imaging X-ray Polarimetry Explorer (IXPE) and the X-ray Imaging and Spectroscopy Mission (XRISM). My work also included optimization and deployment of the balloon-borne X-Calibur experiment for its 2018/2019 long duration balloon flight from McMurdo, Antarctica. Results from this flight allowed me to study the light curve and polarization of the hard X-ray emission from the accreting pulsar GX 301-2. I have also contributed to the research and development for its successor, called XL-Calibur, which will observe pulsars and black holes during a northern hemisphere flight in the next few years

Performance of the X-Calibur Hard X-Ray Polarimetry Mission during its 2018/19 Long-Duration Balloon Flight

X-Calibur is a balloon-borne telescope that measures the polarization of high-energy X-rays in the 15--50keV energy range. The instrument makes use of the fact that X-rays scatter preferentially perpendicular to the polarization direction. A beryllium scattering element surrounded by pixellated CZT detectors is located at the focal point of the InFOC{\mu}S hard X-ray mirror. The instrument was launched for a long-duration balloon (LDB) flight from McMurdo (Antarctica) on December 29, 2018, and obtained the first constraints of the hard X-ray polarization of an accretion-powered pulsar. Here, we describe the characterization and calibration of the instrument on the ground and its performance during the flight, as well as simulations of particle backgrounds and a comparison to measured rates. The pointing system and polarimeter achieved the excellent projected performance. The energy detection threshold for the anticoincidence system was found to be higher than expected and it exhibited unanticipated dead time. Both issues will be remedied for future flights. Overall, the mission performance was nominal, and results will inform the design of the follow-up mission XL-Calibur, which is scheduled to be launched in summer 2022.Comment: 19 pages, 31 figures, submitted to Astropart. Phy