On the large apparent black hole spin-orbit misalignment angle in GW200115

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 $> 90^{\circ}$) 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 $> 90^{\circ}$ in GW200115-like systems implies a kick velocity $\sim 600\, \text{km/s}$ and a kick direction within $\approx 30 ^{\circ}$ of the pre-supernova orbital plane. We discuss different interpretations of the large apparent black hole spin-orbit misalignment angle, including a non-spinning black hole.