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.