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

¹ University of Lodz, Pomorska 149/153, 90-236 Łódź, Poland
² Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
³ Nordita, KTH Royal Institute of Technology and Stockholm University, Hannes Alfvéns väg 12, SE-10691 Stockholm, Sweden
⁴ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, PL-00-716 Warszawa, Poland
⁵ Institute of Physics, University of Zielona Góra, Szafrana 4a, PL-65-516 Zielona Góra, Poland

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 9 March 2026
Accepted: 16 June 2026

Abstract

Aims. We aim to identify the physical mechanism responsible for the observed X-ray emission and polarization of Cyg X-1 in the soft state.

Methods. We performed a detailed spectral analysis of X-ray data obtained with NICER, NuSTAR, and INTEGRAL during observations simultaneous with IXPE on 2023 June 20, supplemented at higher energies with archival CGRO data. We developed a new model, retBB, which describes thermal disk emission and its returning reflection, and applied it together with accurate models of Comptonization and relativistically broadened reflection. Using the resulting spectral solution, we computed the expected polarization signal and compared it with the IXPE measurements.

Results. Our spectropolarimetric modeling shows that the observed polarization is in agreement with being produced by Comptonization in a corona undergoing a semi-relativistic outflow at 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 inconsistent 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.