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

The observational signatures of black holes in x-ray binary systems depend on their masses, spins, accretion rate, and the misalignment angle between the black hole spin and the orbital angular momentum. We present optical polarimetric observations of the black hole x-ray binary MAXI J1820+070, from which we constrain the position angle of the binary orbital. 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° on the spin-orbit 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. Description Misaligned spin in an x-ray binary 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. —KTS A black hole in an x-ray binary system spins in a direction that is misaligned with the binary orbit.