X-ray Polarization changes with the State Transition in Cygnus X--1

Abstract

We studied the polarization properties of Cygnus X-1 in both hard and soft spectral states with Imaging X-ray Polarimetry Explorer (IXPE) observations. The polarization degree is $\sim4$%, and $\sim 2.6-2.8$% in the hard and soft states, respectively. The polarization angle is observed along the jet axis and remains the same in both states. Energy-dependent analysis revealed an increasing polarization with the energy. The observed polarization rules out the lamp-post corona and prefers a conical corona if the coronal geometry remains the same in the hard and soft states. The disk is not detected in the hard state but is found in the soft state in the IXPE spectra. The spectral analysis revealed that the disk emission contributes $\sim 20$% of the total emission in the soft state. The scattering of the seed photons inside the corona is likely to reduce the polarization. This could explain the different polarization in different spectral states. Additionally, if the disk polarization is perpendicular to that of the corona, it could also reduce the polarization in the soft state.