X-ray polarimetry as a new method of studying X-ray binary systems

Abstract

This thesis investigates black hole X-ray binaries through multi-instrument observational data, focusing on spectral and polarimetric analysis. These systems, in which a black hole accretes matter from a companion star, serve as laboratories for studying extreme astrophysical processes. The study combines data from the Imaging X-ray Polarimetry Explorer (IXPE) and complementary X-ray missions to analyze three black hole binaries: Cygnus X-1, Cygnus X-3, and the transient Swift J1727.8–1613. X-ray polarimetry, enabled for the first time with imaging sensitivity by IXPE, has opened a new observational window into the inner regions of accreting black holes. The IXPE campaign on Cygnus X-1 provided the first detailed view of polarization changes across spectral states, constraining the evolving geometry of the corona. For Swift J1727.8–1613, the broadband spectrum during its hard-to-soft state transition was successfully described by a two-phase disk–corona model. In the case of Cygnus X-3, X-ray polarimetry uncovered the unique nature of its emission, shaped by complex scattering environments. These results demonstrate how X-ray polarimetry provides direct insight into the geometry, orientation, and physical conditions of the innermost accretion flows - information previously inaccessible with traditional spectroscopy or timing alone. As a revolutionary tool in high-energy astrophysics, polarimetry significantly improves our ability to study black holes and their vicinity.