  The low energy excitations in Na2IrO3 have been investigated using resonant
inelastic x-ray scattering (RIXS). A magnetic excitation branch can be
resolved, whose dispersion reaches a maximum energy of about 35 meV at the
\Gamma-point. The momentum dependence of the excitation energy is much larger
along the \Gamma-X direction compared to that along the \Gamma-Y direction. The
observed dispersion relation is consistent with a recent theoretical prediction
based on Heisenberg-Kitaev model. At high temperatures, we find large
contributions from lattice vibrational modes to our RIXS spectra, suggesting
that a strong electron-lattice coupling is present in Na2IrO3.
