Correlation functions measured by indirect resonant inelastic X-ray scattering

We express the cross-section for indirect resonant inelastic X-ray scattering in terms of an intrinsic dynamic correlation function of the system that is studied with this technique. We find that the cross-section is a linear combination of the charge response function and the dynamic longitudinal spin density correlation function of the electrons that couple to the core-hole. This result is asymptotically exact for both strong and weak local core-hole potentials. The relative charge and spin contribution to the inelastic spectral weight can be changed by varying the incident photon energy.