Disc and wind in black hole X-ray binary MAXI J1820+070 observed through polarized light during its 2018 outburst

We describe the first complete polarimetric data set 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, similar to 0.15 per cent in B and V filters, and is likely produced in the irradiated disc. A much higher polarization, reaching similar to 0.5 per cent in V and R filters, at a position angle of similar to 25 degrees 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

Black hole spin-orbit misalignment in the x-ray binary MAXI J1820+070

If a black hole is in a close enough binary system with a star, it rips material off the companion. As that material falls into the black hole, it forms an accretion disk that is hot enough to emit optical and x-ray radiation. Poutanen et al. used optical polarimetry to determine the orbital axis of a black hole x-ray binary (see the Perspective by Patat and Mapelli). Combining these observations with previous measurements of the black hole spin showed that the two are misaligned by at least 40 degrees. This high misalignment must have been generated during the formation of the black hole, because accretion always brings the two axes closer together. —KT

Optical polarimetry: Methods, Instruments and Calibration Techniques

In this chapter we present a brief summary of methods, instruments and calibration techniques used in modern astronomical polarimetry in the optical wavelengths. We describe the properties of various polarization devices and detectors used for optical broadband, imaging and spectropolarimetry, and discuss their advantages and disadvantages. The necessity of a proper calibration of the raw polarization data is emphasized and methods of the determination and subtraction of instrumental polarization are considered. We also present a few examples of high-precision measurements of optical polarization of black hole X-ray binaries and massive binary stars made with our DiPol-2 polarimeter, which allowed us to constrain the sources of optical emission in black hole X-ray binaries and measure orbital parameters of massive stellar binaries.Comment: 33 pages, 14 figure; to be published in Astrophysics and Space Science Library 460, Astronomical Polarisation from the Infrared to Gamma Ray