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

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 ${\approx}1$\%, 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, sub-mm 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 $\approx 0.3$\% 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 $\approx-15\deg$, is notably offset from the jet axis, which we interpret as evidence for a misalignment between the black hole spin and the orbital axis.