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

TL;DR

This study provides time-resolved optical polarimetry of Swift J1727.8-1613 during its 2023–2024 outburst, spanning multiple spectral states and radio ejections. After removing interstellar polarization using field stars, the intrinsic optical polarization remains at a subpercent level during the hard-intermediate state and declines sharply at the hard-to-soft transition, with the polarization angle offset by about 15 degrees from jet and X-ray axes. The authors argue that scattering in an optically thin disk wind best explains the observed polarization properties and the PA offset, implying a misalignment between black hole spin and orbital axis. These results constrain wind geometry, accretion flow, and jet interactions in BH X-ray binaries and motivate follow-up polarimetric observations in quiescence to test spin–orbit misalignment.

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 $\approx-15°$, is notably offset from the jet axis, which we interpret as evidence for a misalignment between the black hole spin and the orbital axis.

Figures (10)

• Figure 1: X-ray light and optical curves and X-ray hardness ratio of Swift J1727.8$-$1613. Panel (a): MAXI 2--20 keV X-ray light curve. Panel (b): AAVSO Johnson $V$ filter light curve (https://www.aavso.org). The red arrows indicate the dates of optical spectroscopic detections of the accretion disk winds MataSanchez2024outflows. Panel (c): MAXI 4--10 keV/2--4 keV hardness ratio. Blue, purple, red, and green solid lines indicate dates of optical polarimetric observations through the hard intermediate-, soft intermediate-, soft-, and decaying hard-states of the outburst, respectively.
• Figure 2: Hardness-intensity diagram of Swift J1727.8$-$1613. The gray crosses are from MAXI data. The hardness is taken as the ratio of photon fluxes between 4--10 and 2--4 keV bands. The X-ray photon flux is taken in the 2--20 keV range. The blue circles correspond to the times of optical polarimetric observations. For some optical measurements, the corresponding hardness ratio could not be determined and hence are not shown in the figure. The red triangles denote position of the source during X-ray polarimetric observations by IXPE Podgorny2024.
• Figure 3: Evolution of Swift J1727.8$-$1613 optical polarization and $BVR$ magnitudes around the HIMS/SIMS state transition. The colored symbols correspond to the $B$ (blue triangles), $V$ (green diamonds), and $R$ (red crosses) bands. Panel (a): AAVSO light curve (light green line) and the optical PD in three filters. Panel (b): Observed PA. Panel (c): Optical magnitudes calculated from DIPol-2 images as described in the text. The vertical black dashed line corresponds to MJD 60206, when the jet ejections were detected Wood2025jets.
• Figure 4: Observed normalized Stokes parameters $q$ and $u$ of Swift J1727.8$-$1613 (gray crosses), along with their state-wise averages (colored, open symbols; as marked in the figure) in $B$ (panel a), $V$ (panel b), and $R$ (panel c) bands. The errors on observed data of the source are $1\sigma$. The IS polarization estimate is given by the black circle ($3\sigma$ error) at the intersection of the dotted lines.
• Figure 5: Evolution of the observed PD (panel a) and PA (panel b) in SS and DHS is shown in blue triangles, green diamonds, and red crosses for $B$, $V$, and $R$ bands, respectively.