Recovery of the X-ray polarisation of Swift J1727.8$-$1613 after the soft-to-hard spectral transition

Abstract

We report on the detection of X-ray polarisation in the black-hole X-ray binary Swift J1727.8$-$1613 during its dim hard spectral state by the Imaging X-ray Polarimetry Explorer (IXPE). This is the first detection of X-ray polarisation at the transition from the soft to the hard state in an X-ray binary. We find an averaged 2$-$8 keV polarisation degree of (3.3 $\pm$ 0.4) % and a corresponding polarisation angle of 3° $\pm$ 4°, which matches the polarisation detected during the rising stage of the outburst, in September$-$October 2023, within 1$\sigma$ uncertainty. The observational campaign complements previous studies of this source and enables comparison of the X-ray polarisation properties of a single transient across the X-ray hardness-intensity diagram. The complete recovery of the X-ray polarisation properties, including the energy dependence, came after a dramatic drop in the X-ray polarisation during the soft state. The new IXPE observations in the dim hard state at the reverse transition indicate that the accretion properties, including the geometry of the corona, appear to be strikingly similar to the bright hard state during the outburst rise despite the X-ray luminosities differing by two orders of magnitude.

8 pages, 6 figures. Accepted for publication in A&A Letters, including revision of Fig. 4

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• Hot accretion flow in black hole binaries: a link connecting X-rays to the infrared
• Thermal disc emission from a rotating black hole: X-ray polarization signatures
• Uncovering the geometry of the hot X-ray corona in the Seyfert galaxy NGC4151 with IXPE
• Polarized X-rays from windy accretion in Cygnus X-1
• Dramatic Drop in the X-Ray Polarization of Swift J1727.8$-$1613 in the Soft Spectral State
• NICER, NuSTAR and Insight-HXMT views to the newly discovered black hole X-ray binary Swift J1727.8--1613
• Prospects for differentiating extended coronal geometries in AGNs with the IXPE mission

Cited by in corpus (11)

• Dynamical confirmation of a black hole in the X-ray transient Swift J1727.8-1613
• 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
• Variability of X-ray polarization of Cyg X-1
• Evolution of the Comptonizing medium of the black-hole candidate Swift J1727.8$-$1613 along the hard to hard-intermediate state transition using NICER
• Detection of a Type-C QPO during the soft-to-hard transition in Swift J1727.8-1613
• An Ejection Event Captured by VLBI During the Outburst of Swift J1727.8$-$1613
• Accretion Geometry of Black Hole X-ray Binaries: Insights from X-ray Observations
• Interaction between disk and extended corona in a general relativistic framework
• Optical polarimetry of the accreting black hole X-ray binary Swift J1727.8$-$1613 over the state transition and radio ejections