A unified modeling of X-ray and gamma-ray spectro-polarimetric data: The case of Cygnus X-1

With the advent of new instrumentation, the available spectro-polarimetric measurements in the X-ray and gamma-ray domains have increased significantly. For black-hole X-ray binaries such as Cygnus X-1, a clear discrepancy between the independent measurements of X-ray and soft-γ-ray polarization has emerged. This discrepancy challenges our understanding of the accretion process and emission mechanisms, including acceleration processes at play and the geometry of the system. We thus aim to develop a general-purpose method to model the broadband, high-energy spectro-polarimetric data. Our novel approach makes it possible to simultaneously fit the spectral and polarization data from independent instruments in different energy bands. Measurements with widely different energy bin sizes and statistical significances are combined in a robust and statistically sound way in a single fit, taking into account energy-dependent polarization mixing from different model components. We applied our method to the spectro-polarimetric data of the black-hole X-ray binary Cygnus X-1 in the 2,keV--2,MeV range and show that it enables us to describe the shape of the elusive soft γ-rays' hard tail with unprecedented quality and in exceptional detail. We constrained the photon cutoff energy from optically thin synchrotron emission at ( _ + - ). Using this value, we discuss the origin of the nonthermal electrons in the context of Bohm diffusion and synchrotron cooling. We further provide a new explanation for the misalignment of the polarization of the synchrotron hard tail compared to the jet axis based on magnetic-field helicity and Doppler boosting. Overall, our new approach offers a simple and straightforward way to apply models to diverse spectro-polarimetric data for the broader community. 10^ 2 keV