A1 Vertaisarvioitu alkuperäisartikkeli tieteellisessä lehdessä
Variability of X-ray polarization of Cyg X-1
Tekijät: Kravtsov, Vadim; Bocharova, Anastasiia; Veledina, Alexandra; Poutanen, Juri; Hughes, Andrew K.; Dovčiak, Michal; Egron, Elise; Muleri, Fabio; Podgorny, Jakub; Svoboda, Jiri; Forsblom, Sofia V.; Berdyugin, Andrei V.; Blinov, Dmitry; Bright, Joe S.; Carotenuto, Francesco; Green, David A.; Ingram, Adam; Liodakis, Ioannis; Mandarakas, Nikos; Nitindala, Anagha P.; Rhodes, Lauren; Trushkin, Sergei A.; Tsygankov, Sergey S.; Brigitte, Maimouna; Di Marco, Alessandro; Iacolina, Noemi; Krawczynski, Henric; La Monaca, Fabio; Loktev, Vladislav; Mastroserio, Guglielmo; Petrucci, Pierre-Olivier; Pilia, Maura; Tombesi, Francesco; Zdziarski, Andrzej A.
Kustantaja: EDP SCIENCES S A
Julkaisuvuosi: 2025
Journal: Astronomy and Astrophysics
Artikkelin numero: A115
Vuosikerta: 701
ISSN: 0004-6361
eISSN: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/202555411
Verkko-osoite: https://www.aanda.org/articles/aa/full_html/2025/09/aa55411-25/aa55411-25.html
Rinnakkaistallenteen osoite: https://research.utu.fi/converis/portal/detail/Publication/500446477
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 Pso = 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.
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This research has been supported by the Finnish Cultural Foundation (VK), the Academy of Finland grant 355672 (AV) and the Magnus Ehrnrooth foundation (APN). MD, JPod, and JS acknowledge the support from the Czech Science Foundation project GACR 21–06825X and the institutional support from the Astronomical Institute RVO:6798581. FM, ADM, and FLM are partially supported by MAECI with grant CN24GR08 “GRBAXP: Guangxi-Rome Bilateral Agreement for X-ray Polarimetry in Astrophysics”. AI acknowledges support from the Royal Society. IL was funded by the European Union ERC-2022-STG-BOOTES-101076343. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them. SAT is supported by the Ministry of Science and Higher Education of the Russian Federation grant 075-15-2022-262 (13.MNPMU.21.0003). MB acknowledges the support from GAUK project No. 102323. GM acknowledges financial support from the European Union’s Horizon Europe research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 101107057. POP acknowledges financial support from the Action Thématique PEM from CNRS and from the french spacial agency CNES. AAZ has been supported by the Polish National Science Center grants 2019/35/B/ST9/03944 and 2023/48/Q/ST9/00138.
