  We establish the spectral line shape of orbital excitations created by
resonant inelastic X-ray scattering for the model Mott insulator CoO. Improved
experimental energy resolution reveals that the line shapes are strikingly
different from expectations in a first principles-based atomic multiplet model.
Extended theoretical simulations are performed to identify the underlying
physical origins, which include a pronounced thermal tail reminiscent of
anti-Stokes scattering on the energy gain side of excitations, and an essential
contribution from interatomic many-body dynamics on the energy loss side of
excitations.
