Dramatic Drop in the X-Ray Polarization of Swift J1727.8$-$1613 in the Soft Spectral State

Abstract

Black-hole X-ray binaries exhibit different spectral and timing properties in different accretion states. The X-ray outburst of a recently discovered and extraordinarily bright source, Swift J1727.8$-$1613, has enabled the first investigation of how the X-ray polarization properties of a source evolve with spectral state. The 2$-$8 keV polarization degree was previously measured by the Imaging X-ray Polarimetry Explorer (IXPE) to be $\approx$ 4% in the hard and hard intermediate states. Here we present new IXPE results taken in the soft state, with the X-ray flux dominated by the thermal accretion-disk emission. We find that the polarization degree has dropped dramatically to $\lesssim$ 1%. This result indicates that the measured X-ray polarization is largely sensitive to the accretion state and the polarization fraction is significantly higher in the hard state when the X-ray emission is dominated by up-scattered radiation in the X-ray corona. The combined polarization measurements in the soft and hard states disfavor a very high or low inclination of the system.

9 pages, 7 figures

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Cited by in corpus (13)

• Dynamical confirmation of a black hole in the X-ray transient Swift J1727.8-1613
• Recovery of the X-ray polarisation of Swift J1727.8$-$1613 after the soft-to-hard spectral transition
• X-ray polarization from accretion disk winds
• Ultrasoft state of microquasar Cygnus X-3: X-ray polarimetry reveals the geometry of astronomical puzzle
• A broadband spectral-timing study of QPOs in the bright black hole X-ray binary Swift J1727.8-1613
• {\it Insight}-HXMT View of the BHC Swift J1727.8-1613 during its outburst in 2023
• Variability of X-ray polarization of Cyg X-1
• Comprehensive Radio Monitoring of the Black Hole X-ray Binary Swift J1727.8$-$1613 during its 2023$-$2024 Outburst
• Detection of a Type-C QPO during the soft-to-hard transition in Swift J1727.8-1613
• Investigating the consistency of the shape and flux of X-ray reflection spectra in the hard state with an accretion disk reaching close to the black hole
• An Ejection Event Captured by VLBI During the Outburst of Swift J1727.8$-$1613
• Accretion Geometry of Black Hole X-ray Binaries: Insights from X-ray Observations
• Optical polarimetry of the accreting black hole X-ray binary Swift J1727.8$-$1613 over the state transition and radio ejections