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 summarize key observational constraints of the accretion flow on the black hole X-ray binary Cygnus X-1 (Cyg X-1). The discussion highlights the flows of energy close to the black hole and the importance of the distance range from which the radiating zone draws its energy. For the hard state, we examine compact and extended corona models. We find that compact corona models are energetically favored, but extended models cannot be fully excluded. We discuss the high linear polarization of the Cyg X-1 X-rays in the soft and hard states, parallel to the direction of the radio jet. We propose the presence of a pair layer enveloping the accretion disk moving at approximately half the speed of light away from the disk for both the soft and the hard state. In the soft state, the pairs cool to the Compton temperature of the disk emission. In the hard state, the pairs acquire thermal and bulk motion allowing them to Comptonize the emission to produce the observed power law emission. In both emission states, the bulk motion away from the disk leads to a net polarization parallel to the radio jet. We emphasize that the geometry of the accretion flow in the hard state is still not well constrained, and that observed spectral (including the relativistically broadened Fe K-$\alpha$ line) and spectro-polarimetric signatures depend strongly on the plasma processes responsible for energy dissipation in the plasma.