Unchanged X-Ray Polarization during Accretion Dips in the Low Hard State of Cygnus X-1
- 1Guangxi University
- 2INAF Istituto di Astrofisica e Planetologia Spaziali
- 3Key Laboratory of Particle Astrophysics
Abstract
Cygnus X-1 serves as a foundational benchmark for studying accretion mechanisms in stellar-mass black hole X-ray binaries. While traditional X-ray timing and spectroscopy have mapped its bimodal state transitions, constraining the exact geometric configuration of its accreting plasma requires additional tools. In this article, we present a joint spectropolarimetric analysis of Cygnus X-1 during flux-decrease events, or “dips”, observed in its low hard state using simultaneous data from IXPE, NICER, and NuSTAR. The broadband spectral model reveals that the energy-dependent flux reduction during these dips is best described by a partial-covering absorption model, heavily impacting the softer energy bands. This absorption is consistent with obscuration by dense, cold clumps originating from the companion’s stellar wind or the outer accretion disk during superior conjunction. Notably, our polarimetric analysis demonstrates that despite significant flux drops, the polarization degree and polarization angle in the 2–8 keV band remain largely consistent between dip and off-dip intervals. The stability of the polarization signature during these highly absorbed periods indicates that the obscured thermal disk emission does not significantly contribute to the total polarization. Ultimately, these results strongly support a scenario where the X-ray polarization originates from an extended, likely oblate, accretion-disk corona whose large scale renders its overall geometry unaffected by localized clumping.
Related
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BibTeX
@article{ling2026unchanged,
title = {{Unchanged X-Ray Polarization during Accretion Dips in the Low Hard State of Cygnus X-1}},
author = {Yu-Shan Ling and Fei Xie and Alessandro Di Marco and Fabio La Monaca and Ming-Yu Ge},
journal = {The Astrophysical Journal},
year = {2026},
month = nov,
doi = {10.3847/1538-4357/aea6d4},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea6d4},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Unchanged X-Ray Polarization during Accretion Dips in the Low Hard State of Cygnus X-1
AU - Yu-Shan Ling
AU - Fei Xie
AU - Alessandro Di Marco
AU - Fabio La Monaca
AU - Ming-Yu Ge
PY - 2026
DA - 2026/11/03
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/aea6d4
UR - https://iopscience.iop.org/article/10.3847/1538-4357/aea6d4
AB - Cygnus X-1 serves as a foundational benchmark for studying accretion mechanisms in stellar-mass black hole X-ray binaries. While traditional X-ray timing and spectroscopy have mapped its bimodal state transitions, constraining the exact geometric configuration of its accreting plasma requires additional tools. In this article, we present a joint spectropolarimetric analysis of Cygnus X-1 during flux-decrease events, or “dips”, observed in its low hard state using simultaneous data from IXPE, NICER, and NuSTAR. The broadband spectral model reveals that the energy-dependent flux reduction during these dips is best described by a partial-covering absorption model, heavily impacting the softer energy bands. This absorption is consistent with obscuration by dense, cold clumps originating from the companion’s stellar wind or the outer accretion disk during superior conjunction. Notably, our polarimetric analysis demonstrates that despite significant flux drops, the polarization degree and polarization angle in the 2–8 keV band remain largely consistent between dip and off-dip intervals. The stability of the polarization signature during these highly absorbed periods indicates that the obscured thermal disk emission does not significantly contribute to the total polarization. Ultimately, these results strongly support a scenario where the X-ray polarization originates from an extended, likely oblate, accretion-disk corona whose large scale renders its overall geometry unaffected by localized clumping.
SN - 1538-4357
SN - 0004-637X
ER -