Abstract
The recent launches of the {\it Imaging X-ray Polarimetry Explorer} ({\it IXPE}\,) and {\it XRISM} have provided significant advances to the realms of polarization and high-resolution spectroscopy in X-ray astrophysics. However, both of these satellite missions conduct measurements in the soft X-rays, below 10 keV. To obtain a complete picture of any astrophysical source, we must extend these capabilities to the hard X-rays, above 10 keV, as well. Thus, in my graduate work, I have directed my efforts in these two regimes: measuring the polarization of hard X-rays with {\it XL-Calibur}, and improving the energy resolution of hard X-ray telescopes with {\it DR-TES}---both balloon-borne missions. In this work, I describe the most precise polarization measurement made so far of a black hole X-ray binary (BHXRB) in the hard X-rays, with {\it XL-Calibur}. The results of this mission pave the way to more fully understanding the BHXRB corona, dominant in this energy range, as well as fill in the hard-X ray gap in the broadband polarization picture of BHXRBs provided by {\it IXPE} and the upcoming gamma-ray {\it Compton Spectrometer and Imager} ({\it COSI\,}). I describe also our efforts towards flying a mini dilution refrigerator on a balloon for the first time, as well as flying transition-edge sensors (TESs) for X-rays on a balloon for the first time, with {\it DR-TES}. Its successful flight thus sets the stage for future missions to study emission lines from supernovae and the Galactic Center with unprecedented energy resolution. In addition to these two main missions, work done i) to constrain axion-like particles using the soft X-ray polarization of magnetars observed by {\it IXPE} as well as ii) to propose a novel magnetic readout (using nitrogen-vacancy centers) of magnetic microcalorimeters (which, along with TESs, offer one of the best energy resolutions to date) will be described.
Committee Chair
Henric Krawczynski
Committee Members
Bhupal Dev; Christine Done; Kater Murch; Manel Errando
Degree
Doctor of Philosophy (PhD)
Author's Department
Physics
Document Type
Dissertation
Date of Award
4-21-2026
Language
English (en)
DOI
https://doi.org/10.7936/qjkp-zz28
Author's ORCID
https://orcid.org/0000-0002-5250-2710
Recommended Citation
Gau, Ephraim, "Development and Flight of Stratospheric Missions for Hard X-Ray Polarimetry and Hard X-Ray Spectroscopy" (2026). Arts & Sciences Graduate Student Theses and Dissertations. 3746.
The definitive version is available at https://doi.org/10.7936/qjkp-zz28