On the Large Apparent Black Hole Spin-orbit Misalignment Angle in GW200115

Abstract

Abstract GW200115 is one of the first two confidently detected gravitational-wave events of neutron star–black hole mergers. An interesting property of this merger is that the black hole, if spinning rapidly, has its spin axis negatively aligned (with a misalignment angle >90°) with the binary orbital angular momentum vector. Although such a large spin–orbit misalignment angle naturally points toward a dynamical origin, the measured neutron star–black hole merger rate exceeds theoretical predictions of the dynamical formation channel. In the canonical isolated binary formation scenario, the immediate progenitor of GW200115 is likely to be a binary consisting of a black hole and a helium star, with the latter forming a neutron star during a supernova explosion. Since the black hole is generally expected to spin along the pre-supernova binary orbital angular momentum axis, a large neutron star natal kick is required to produce the observed misalignment angle. Using simple kinematic arguments, we find that a misalignment angle >90° in GW200115-like systems implies a kick velocity ∼600 km s−1 and a kick direction within ≈30° of the pre-supernova orbital plane. We discuss different interpretations of the large apparent black hole spin–orbit misalignment angle, including a nonspinning black hole.