Variability of X-ray polarization of Cyg X-1

¹ Department of Physics and Astronomy, FI-20014 University of Turku, Finland
² Nordita, KTH Royal Institute of Technology and Stockholm University, Hannes Alfvéns väg 12, SE-10691 Stockholm, Sweden
³ Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁴ Astronomical Institute of the Czech Academy of Sciences, Boční II 1401/1, 14100 Praha 4, Czech Republic
⁵ INAF Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
⁶ INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁷ Institute of Astrophysics, FORTH, N. Plastira 100, GR-70013 Heraklion, Greece
⁸ Department of Physics, University of Crete, GR-70013 Heraklion, Greece
⁹ INAF Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone (RM), Italy
¹⁰ Astrophysics Group, Cavendish Laboratory, 19 J. J. Thomson Avenue, Cambridge CB3 0HE, UK
¹¹ School of Mathematics, Statistics, and Physics, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
¹² Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhnij Arkhyz, 369167 Karachayevo-Cherkessia, Russia
¹³ Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, Prague 8 18000, Czech Republic
¹⁴ Agenzia Spaziale Italiana, via della Scienza 5, 09047 Selargius (CA), Italy
¹⁵ Physics Department, McDonnell Center for the Space Sciences, and Center for Quantum Leaps, Washington University in St. Louis, St. Louis, MO 63130, USA
¹⁶ Dipartimento di Fisica, Università degli Studi di Roma ‘Tor Vergata’, Via della Ricerca Scientifica 1, 00133 Rome, Italy
¹⁷ Department of Physics, P.O. Box 64 FI-00014 University of Helsinki, Finland
¹⁸ Dipartimento di Fisica, Università Degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
¹⁹ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
²⁰ INAF Astronomical Observatory of Rome, Via Frascati 33, 00040 Monte Porzio Catone, Italy
²¹ INFN Sezione di Roma ‘Tor Vergata’, Via della Ricerca Scientifica 1, 00133 Rome, Italy
²² Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, PL-00-716 Warszawa, Poland

^⋆ Corresponding author: vakrau@utu.fi

Received: 6 May 2025
Accepted: 24 July 2025

Abstract

We present the results of a three-year X-ray, optical, and radio polarimetric monitoring campaign of the prototypical black hole X-ray binary Cyg X-1, conducted from 2022 to 2024. The X-ray polarization of Cyg X-1 was measured 13 times with the Imaging X-ray Polarimetry Explorer (IXPE), covering both hard and soft spectral states. The X-ray polarization degree (PD) in the hard state was found to be ≈4.0%, roughly twice as high as in the soft state, where it was around 2.2%. In both states, a statistically significant increase in PD with the energy was found. Moreover, a linear relation between PD and spectral hardness suggests a gradual and continuous evolution of the polarization properties, rather than an abrupt change of polarization production mechanism between states. The polarization angle (PA) was independent of the spectral state and showed no trend with the photon energy. The X-ray PA is well aligned with the orientation of the radio jet, as well as the optical and radio PAs. We find significant orbital changes of PA in the hard state, which we attribute to scattering of X-ray emission at the intrabinary structure. No significant superorbital variability in PD or PA was found at the period P_so = 294 d. We detect, for the first time in this source, polarization of the radio emission, with the PA aligned with the jet, and a strong increase of the PD at a transition to the soft state. We also find no correlation between the X-ray and optical polarization; if any, there is a long-term anti-correlation between the X-ray PD and the radio PD.