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

Abstract

New X-ray polarization 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 polarization is much higher than predicted. This system also shows a strong, highly ionized wind, consistent with thermal-radiative driving from the outer disc, leading to speculation that scattering in the wind is responsible for the unexpectedly high polarization degree from a standard optically thick disc. Here, we show that this is not the case. The optically thin(ish) wind polarizes the scattered light in a direction orthogonal to that predicted from a standard optically thick disc, reducing about 2 per cent rather than enhancing the predicted polarization of the total emission. This value is consistent with the polarization 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 polarization direction must be orthogonal to the disc plane rather than parallel as expected from optically thick material.

X-Ray Polarization of the Black Hole X-Ray Binary 4U 1630–47 Challenges the Standard Thin Accretion Disk Scenario

“…Further, for the reflected flux to make a significant contribution to the total spectrum, the solid angle subtended by the wind to the X-ray source needs to be large; in this case, emission lines should be present, which we do not see in the NICER and NuSTAR energy spectra. Recently, Tomaru et al (2024) studied a possible wind contribution to the polarization and concluded that the wind cannot explain the very high PDs observed in this source. Finally, in a subsequent observation of the source, when it exhibited an SPL state and no evidence for a wind was apparent, the PD and its energy dependence were similar to the high-soft state (Rodriguez Cavero et al 2023), again arguing against a wind origin for the polarization.…”