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

TL;DR

This study uses optical polarimetry to measure the spin-orbit misalignment in the black hole X-ray binary MAXI J1820+070, revealing a significant misalignment that impacts black hole property measurements.

Contribution

First direct constraint on the spin-orbit misalignment angle in MAXI J1820+070 using optical polarimetry and jet orientation data.

Findings

Lower limit of 40 degrees on the misalignment angle.

Misalignment likely originates from binary or black hole formation.

Potential bias in black hole mass and spin measurements from X-ray data.

Abstract

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 axis. Combining this with previous determinations of the relativistic jet orientation axis, which traces the black hole spin, and the inclination of the orbit, we determine a lower limit of 40 deg on the spin-orbit misalignment angle. The misalignment has to originate from either the binary or black hole formation stage. If other x-ray binaries have similarly large misalignments, these would bias measurements of black hole masses and spins from x-ray observations.