Evolving optical polarisation of the black hole X-ray binary MAXI J1820+070

TL;DR

This study measures optical polarisation in the black hole binary MAXI J1820+070 during outburst, revealing small but significant intrinsic polarisation changes linked to accretion flow and jet contributions.

Contribution

First polarimetric monitoring of MAXI J1820+070 during outburst, providing insights into the origin of optical emission and magnetic field geometry near the black hole.

Findings

Detected intrinsic polarisation of 0.3-0.7% in MAXI J1820+070.

Observed polarisation increase after April 14, 2018, correlated with flux drop.

Suggests polarisation arises from irradiated disc or scattering in outflow.

Abstract

Aims. The optical emission of black hole transients increases by several magnitudes during the X-ray outbursts. Whether the extra light arises from the X-ray heated outer disc, from the inner hot accretion flow, or from the jet is currently debated. Optical polarisation measurements are able to distinguish the relative contributions of these components. Methods. We present the results of BVR polarisation measurements of the black hole X-ray binary MAXI J1820+070 during the period of March-April 2018. Results. We detect small, $\sim$0.7%, but statistically significant polarisation, part of which is of interstellar origin. Depending on the interstellar polarisation estimate, the intrinsic polarisation degree of the source is between $\sim$0.3% and 0.7%, and the polarisation position angle is between $\sim10\deg-30\deg$. We show that the polarisation increases after MJD 58222 (2018 April 14). The change is of the order of 0.1% and is most pronounced in the R band. The change of the source Stokes parameters occurs simultaneously with the drop of the observed V-band flux and a slow softening of the X-ray spectrum. The Stokes vectors of intrinsic polarisation before and after the drop are parallel, at least in the V and R filters. Conclusions. We suggest that the increased polarisation is due to the decreasing contribution of the non-polarized component, which we associate with the the hot flow or jet emission. The low polarisation can result from the tangled geometry of the magnetic field or from the Faraday rotation in the dense, ionised, and magnetised medium close to the black hole. The polarized optical emission is likely produced by the irradiated disc or by scattering of its radiation in the optically thin outflow.