X-ray polarisation properties of thermal-radiative disc winds in binary systems

Abstract

New X-ray polarisation results are challenging our understanding of the accretion flow geometry in black hole binary systems. Even spectra dominated by a standard disc can give unexpected results, such as the high inclination black hole binary 4U 1630- 472, where the observed X-ray polarisation is much higher than predicted. This system also shows a strong, highly ionised wind, consistent with thermal-radiative driving from the outer disc, leading to speculation that scattering in the wind is responsible for the unexpectedly high polarisation degree from a standard optically thick disc. Here we show that this is not the case. The optically thin(ish) wind polarises the scattered light in a direction orthogonal to that predicted from a standard optically thick disc, reducing about 2% rather than enhancing the predicted polarisation of the total emission. This value is consistent with the polarisation difference between the disc-dominated soft state, where absorption lines by the wind are clearly seen, and the steep power-law state, where no absorption lines are seen. If this difference is genuinely due to the presence or absence of wind, the total polarisation direction must be orthogonal to the disc plane rather than parallel as expected from optically thick material.

8 pages, 10 figures, accepted by MNRAS

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Cited by in corpus (13)

• X-ray Polarization of the Black Hole X-ray Binary 4U 1630-47 Challenges Standard Thin Accretion Disk Scenario
• X-ray polarization from accretion disk winds
• Discovery of a strong rotation of the X-ray polarization angle in the galactic burster GX 13+1
• New polarimetric study of the galactic X-ray burster GX 13+1
• Polarized radiation from the spreading layer of the weakly magnetized neutron stars
• X-ray spectro-polarimetric characterization of GX 340+0 in the horizontal branch: a highly inclined source?
• X-ray Dips and Polarization Angle Swings in GX 13+1
• Accretion disc winds in X-ray binaries
• X-ray spectropolarimetric characterization of the Z-source GX 340+0 in the normal branch
• IXPE view of the Sco-like source GX 349+2 in the normal branch
• First Detection of X-ray Polarization and Its Short-term Increase Pre- and Post-Eclipse in HMXB 4U 1700-377
• The detection of high X-ray polarization from an accretion disc corona source and its modelling via Monte Carlo radiation transfer simulation
• Shape and ionization of equatorial matter near compact objects from X-ray polarization reflection signatures