Recent X-ray polarimetric data on the prototypical black hole X-ray binary Cyg~X-1 from the Imaging X-ray Polarimetry Explorer present tight constraints on accretion geometry in the hard spectral state. Contrary to general expectations of a low, <1% polarization degree (PD), the observed average PD was found to be a factor of 4 higher. Aligned with the jet position angle on the sky, the observed polarization favors geometry of the X-ray emission region stretched normally to the jet in the accretion disk plane. The high PD is, however, difficult to reconcile with the low orbital inclination of the binary i≈30 deg. We suggest that this puzzle can be explained if the emitting plasma is outflowing with a mildly relativistic velocity ≳0.4c. Our radiative transfer simulations show that Comptonization in the outflowing medium elongated in the plane of the disk radiates X-rays with the degree and direction of polarization consistent with observations at i≈30 deg.
@article{arxiv.2302.11674,
title = {Polarized X-rays from windy accretion in Cygnus X-1},
author = {Juri Poutanen and Alexandra Veledina and Andrei M. Beloborodov},
journal= {arXiv preprint arXiv:2302.11674},
year = {2023}
}