Spin flips and precession in black-hole-binary mergers

We use the ``moving puncture'' approach to perform fully nonlinear evolutions of spinning quasicircular black-hole binaries with individual spins unaligned with the orbital angular momentum. We evolve configurations with the individual spins (parallel and equal in magnitude) pointing in the orbital plane and 45\ifmmode^\circ\else\textdegree\fi{} above the orbital plane. We introduce a technique to measure the spin direction and track the precession of the spin during the merger, as well as measure the spin flip in the remnant horizon. The former configuration completes 1.75 orbits before merging, with the spin precessing by 98\ifmmode^\circ\else\textdegree\fi{} and the final remnant horizon spin flipped by $\ensuremath{\sim}72\ifmmode^\circ\else\textdegree\fi{}$ with respect to the component spins. The latter configuration completes 2.25 orbits, with the spins precessing by 151\ifmmode^\circ\else\textdegree\fi{} and the final remnant horizon spin flipped $\ensuremath{\sim}34\ifmmode^\circ\else\textdegree\fi{}$ with respect to the component spins. These simulations show for the first time how the spins are reoriented during the final stage of black-hole-binary mergers verifying the hypothesis of the spin-flip phenomenon. We also compute the track of the holes before merger and observe a precession of the orbital plane with frequency similar to the orbital frequency and amplitude increasing with time.