  We consider the low-energy excitations of one-dimensional spin-orbital models
which consist of spin waves, orbital waves, and joint spin-orbital excitations.
Among the latter we identify strongly entangled spin-orbital bound states which
appear as peaks in the von Neumann entropy (vNE) spectral function introduced
in this work. The strong entanglement of bound states is manifested by a
universal logarithmic scaling of the vNE with system size, while the vNE of
other spin-orbital excitations saturates. We suggest that spin-orbital
entanglement can be experimentally explored by the measurement of the dynamical
spin-orbital correlations using resonant inelastic x-ray scattering, where
strong spin-orbit coupling associated with the core hole plays a role.
