The Cygnus X-1 Puzzle: Implications of X-ray Polarization Measurements in the Soft and Hard States on the Properties of the Accretion Flow and the Emission Mechanisms

Abstract: In this paper, we explore the implications of the recent X-ray polarization observations of the X-ray Black Hole Binary (XRBHB) Cygnus X-1 (Cyg X-1) with IXPE and XL-Calibur. The discussion highlights the importance of the distance range from which the radiating zone draws its energy, and of the mechanism that transports the energy from the accretion disk into the radiating zone. We examine compact and extended corona models, finding that the former are energetically favored, but the latter cannot yet be fully excluded. Finally, the high linear polarization of the Cyg X-1 X-rays in the soft and in the hard states, parallel to the direction of the radio jet, is addressed. A scenario for the soft state is proposed in which shear stresses and magnetic reconnection create a pair layer enveloping the accretion disk, with the pairs accelerating due to radiation pressure from the accretion disk to mildly relativistic velocities. Comptonization by these pairs at roughly the same temperature as the accretion disk's radiation can explain the polarization properties without major modifications of the emitted energy spectrum. We emphasize that the geometry of the accretion flow in the hard state is still not well constrained, and that the polarization results depend sensitively on plasma processes responsible for energy dissipation in the plasma.