  We investigate the Cu $L_3$ edge resonant inelastic x-ray scattering (RIXS)
spectra of a quasi-1D antiferromagnet Ca$_2$CuO$_3$. In addition to the
magnetic excitations, which are well-described by the two-spinon continuum, we
observe two dispersive orbital excitations, the $3d_{xy}$ and the $3d_{yz}$
orbitons. We carry out a quantitative comparison of the RIXS spectra, obtained
with two distinct incident polarizations, with a theoretical model. We show
that any realistic spin-orbital model needs to include a finite, but realistic,
Hund's exchange $J_H \approx 0.5$ eV. Its main effect is an increase in orbiton
velocities, so that their theoretically calculated values match those observed
experimentally. Even though Hund's exchange also mediates some interaction
between spinon and orbiton, the picture of spin-orbit separation remains intact
and describes orbiton motion in this compound.
