Speaker
Description
The soft state spectrum of X-ray binaries exhibits a high-energy tail extending beyond 500 keV that cannot be explained by standard thermal disk emission models alone. The launch of IXPE has enabled X-ray polarimetry, which is sensitive to system geometry and can therefore provide new constraints on viewing angle and corona configuration. We aim to develop a self-consistent spectro-polarimetric model of Cyg X-1's soft state that simultaneously describes the broadband spectrum and polarization properties, and to use polarimetric measurements to derive independent constraints for the system's geometry.
We adopt a hybrid Comptonization scenario with slab corona geometry, in which soft disk photons are up-scattered in a hot corona while hard X-ray photons irradiate the disk surface, increasing the soft photon supply. We fit the model to simultaneous observations from IXPE, NuSTAR, and INTEGRAL obtained in June 2023, supplemented by archival OSSE data from 1994, covering energy band from 2 keV to 1 MeV. Our model successfully reproduces the entire broadband spectrum, including the high-energy tail, as well as the observed polarization properties. By comparing model predictions of polarization degree as a function of orbital inclination angle with observations, we constrain the system's orbital inclination angle.