X-ray polarization in the soft state of Cyg X-1

TL;DR

This study combines spectral and polarization data to identify that Comptonization in a semi-relativistic outflowing corona explains the X-ray polarization in Cyg X-1's soft state, favoring a low black-hole spin.

Contribution

The paper introduces a new spectral model, retBB, and demonstrates how combined spectral and polarization analysis constrains the physical origin of X-ray polarization in Cyg X-1.

Findings

Polarization is produced by Comptonization in a semi-relativistic corona.

Observed polarization strongly favors a low black-hole spin.

General relativistic effects and returning disk radiation are minor in polarization production.

Abstract

We aim to identify the physical mechanism responsible for the observed X-ray emission and polarization of Cyg X-1 in the soft state. We perform a detailed spectral analysis of X-ray data obtained by NICER, NuSTAR, and INTEGRAL during observations simultaneous with IXPE on 2023 June 20, supplemented at higher energies with archival CGRO data. We develop a new model, retBB, that describes thermal disk emission and its returning reflection, and apply it together with accurate models of Comptonization and relativistically broadened reflection. Using the resulting spectral solution, we compute the expected polarization signal and compare it with the IXPE measurements. Our spectropolarimetric modeling shows that the observed polarization is produced by Comptonization in a corona undergoing a semi-relativistic outflow with a velocity of ~0.3c. Our spectral solutions admit either low or high black-hole spin values, depending on the adopted model setup. However, the observed polarization strongly favors a low spin. At high spin, the polarization angle would inevitably rotate significantly across the energy band, which is not consistent with the observations. Apart from this rotation of the polarization angle, general relativistic effects do not play a significant role in producing the observed polarization. In particular, we find that, at most, returning disk radiation contributes only a minor amount.