Disc and wind in black hole X-ray binary MAXIJ1820+070 observed through polarized light during its 2018 outburst

TL;DR

This study presents the first complete polarimetric dataset of a black hole X-ray binary outburst, revealing polarization changes linked to different states and winds, constraining the system's geometry and emission mechanisms.

Contribution

It provides the first comprehensive polarimetric observations of MAXIJ1820+070 during its outburst, linking polarization features to system geometry and wind phenomena.

Findings

Polarization during the soft state is low and likely from the irradiated disc.

Higher polarization during the hard state coincides with wind detection and aligns with the jet angle.

Optical polarization is best explained by scattering in an equatorial wind.

Abstract

We describe the first complete polarimetric dataset of the entire outburst of a low-mass black hole X-ray binary system and discuss the constraints for geometry and radiative mechanisms it imposes. During the decaying hard state, when the optical flux is dominated by the non-thermal component, the observed polarization is consistent with the interstellar values in all filters. During the soft state, the intrinsic polarization of the source is small, $\sim 0.15$ per cent in $B$ and $V$ filters, and is likely produced in the irradiated disc. A much higher polarization, reaching $\sim 0.5$ per cent in $V$ and $R$ filters, at position angle of $\sim 25^\circ$ observed in the rising hard state coincides in time with the detection of winds in the system. This angle coincides with the position angle of the jet. The detected optical polarization is best explained by scattering of the non-thermal (hot flow or jet base) radiation in an equatorial wind.