Optical polarimetry of the accreting black hole X-ray binary Swift J1727.8-1613 over the state transition and radio ejections;
: Nitindala, Anagha P.; Veledina, Alexandra; Kravtsov, Vadim; Berdyugin, Andrei V.; Diaz Teodori, Maria Alejandra; Piirola, Vilppu; Sakanoi, Takeshi; Kagitani, Masato; Berdyugina, Svetlana V.; Poutanen, Juri
Publisher: EDP Sciences
: 2026
Astronomy and Astrophysics
: A184
: 709
: 0004-6361
: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/202558488
: https://www.aanda.org/articles/aa/full_html/2026/05/aa58488-25/aa58488-25.html
: https://research.utu.fi/converis/portal/detail/Publication/523971914
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.
stars: black holes
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This research has been supported by the Magnus Ehrnrooth Foundation (APN) and the Academy of Finland grant 355672 (AV). Nordita is supported in part by NordForsk. The DIPOL-2 was built in cooperation by the University of Turku, Finland, and the Kiepenheuer Institut fuer Sonnenphysik, Germany, with support from the Leibniz Association grant SAW-2011-KIS-7. We are grateful to the Institute for Astronomy, University of Hawaii for the observing time allocated for us on the T60 telescope.The data collected using DIPol-UF were based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. The NOT data were obtained under program ID P71-015. We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This research has made use of “Aladin sky atlas” developed at CDS, Strasbourg Observatory, France (Baumann et al. 2022).
