Optical polarimetry of the accreting black hole X-ray binary Swift J1727.8−1613 over the state transition and radio ejections

¹ Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
² Nordita, Stockholm University and KTH Royal Institute of Technology, Hannes Alfvéns väg 12, SE-10691 Stockholm, Sweden
³ Graduate School of Sciences, Tohoku University, Aoba-ku, 980-8578 Sendai, Japan
⁴ Istituto ricerche solari Aldo e Cele Daccó (IRSOL), Faculty of Informatics, Università della Svizzera italiana, Via Patocchi 57, Locarno, Switzerland

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Received: 9 December 2025
Accepted: 7 March 2026

Abstract

We present the first optical (BVR) polarimetric observations of Swift J1727.8−1613 during its 2023–2024 outburst. Observations were performed during the X-ray hard-to-soft state transition, the soft state, and the decaying hard state of the source. For the vast majority of nights, we detect statistically significant polarization of ≈1%, a fraction of which is of interstellar origin. We find a significant change of polarization coinciding in time with discrete radio ejections. The direction of this polarization variation differs from the directions inferred from the X-ray, submillimeter, and radio polarization angles, as well as from the resolved jet orientation. After correcting for the interstellar component, we find that the intrinsic polarization degree remained approximately constant at PD ≈0.3% throughout the hard-intermediate state. We explore several possible origins for the polarization and conclude that it is most plausibly produced by scattering within the optically thin accretion disk wind. The intrinsic polarization angle, PA ≈ − 15°, is notably offset from the jet axis, which we interpret as evidence of a misalignment between the black hole spin and the orbital axis.