Due to the advent of a new generation of detectors, X-ray polarimetry promises to join X-ray imaging, spectroscopy and timing as one of the main observational techniques in high energy astrophysics. This has renewed interest in the field, and indeed several polarimetric missions have recently been proposed. This volume provides a complete and up-to-date view of the subject for researchers in astrophysics. The contributors discuss the present status and perspectives of instruments, review current theoretical models, and examine future missions. As well as detailed papers, the book contains broad reviews that can be easily understood by astrophysicists new to the field.

Due to the advent of a new generation of detectors, X-ray polarimetry promises to join X-ray imaging, spectroscopy and timing as one of the main observational techniques in high energy astrophysics. This has renewed interest in the field, and indeed several polarimetric missions have recently been proposed. This volume provides a complete and up-to-date view of the subject for researchers in astrophysics. The contributors discuss the present status and perspectives of instruments, review current theoretical models, and examine future missions. As well as detailed papers, the book contains broad reviews that can be easily understood by astrophysicists new to the field.

Measuring X-ray polarization has been a major goal in astrophysics for the last 40 years. With the launch of NASA's Imaging X-ray Polarimetry Explorer (IXPE) in December 2021, soft X-ray polarimetry (1-10 keV) is now possible. In this thesis, I introduce an optimal signal extraction method for X-ray polarimeters based on gas pixel detectors. Using IXPE's thus improved sensitivity, I make progress towards identifying the high-energy emission processes and magnetic field geometries responsible for radiation in blazar jets. First, I develop a simulation-based inference framework for measuring X-ray polarization as a function of time, energy, and position in gas pixel detectors, proving that for a fixed model budget the framework is optimal. Second, I propose a simple geometrical jet model to study the polarization properties of leptonic blazar jets, including their observed EVPA rotations and their detection prospects with IXPE. Finally, I interpret IXPE's measurements of BL Lacertae across the synchro-Compton transition region to suggest leptonic-dominated emission processes in blazars, and of Mrk 421 to provide evidence for helical magnetic fields as the source of EVPA rotations.

"The polarization study of celestial objects is a valuable part of optical astronomy, and the author has done exceptionally well in bringing to gether contributions treating all aspects of the polarimetry field. . . . The first section contains a fine introduction and an excellent and definitive history of the subject. . . . The volume is well illustrated. . . . Highly recommended."ÑChoice "The high quality of this book is clearly due to strict editorial attention to each paper and the discussions. Gehrel's book will surely stand for many years as the fundamental reference source for polarization studies in astronomy as well as in atmospheric physics."ÑJournal of the Assoc. of Lunar and Planetary Observers