Optical polarimetry as a tool to learn orbital orientation and nature of black hole X-ray binaries

Rome Joint Astrophysics Colloquium - Juri Poutanen (University of Turku) - High-precision optical polarimetry as a tool to learn about orbital orientation and nature of optical emission of black hole X-ray binaries - 17/10/2022
The Rome JAC is a joint initiative of the Physics Dept. of the Tor Vergata University of Rome, INAF - Astronomical Observatory of Rome, and INAF - Institute for Space Astrophysics and Planetology.
Abstract
The emission region and physical mechanisms producing radiation in many astrophysical objects, including stellar-mass X-ray binary systems. I present the results of multiwavelength (BVR) polarimetric studies of a sample of historical black hole X-ray binaries, observed during their outbursts or in the quiescent state. As a highlight I will concentrate on the analysis and interpretation of the data on black hole X-ray binary MAXI J1820+070 obtained with high-precision polarimeter DIPol-UF at the Nordic Optical Telescope, from which we constrain the position angle of the binary orbital axis. Combining this with previous determinations of the relativistic jet orientation, which traces the black hole spin, and the inclination of the orbit, we determine a lower limit of 40 deg on the black hole spin-orbital angular momentum misalignment angle. The misalignment must originate from either the binary evolution or black hole formation stages. If other X-ray binaries have similarly large misalignments, these would bias measurements of black hole masses and spins from X-ray observations. A high misalignment adds complexity to the models of the X-ray and optical quasi-periodic oscillations observed from black hole X-ray binaries in their hard state.