  We study the magnetic excitation spectra of resonant inelastic x-ray
scattering (RIXS) at the $L$-edge from undoped cuprates beyond the fast
collision approximation. We analyse the effect of the symmetry breaking ground
state on the RIXS process of the Heisenberg model by using a projection
procedure. We derive the expressions of the scattering amplitude in both
one-magnon and two-magnon excitation channels. Each of them consists of the
isotropic and anisotropic contributions. The latter is a new finding and
attributed to the long range order of the ground state. The presence of
anisotropic terms is supported by numerical calculations on a two-dimensional
spin cluster. We express the RIXS spectra in the form of spin-correlation
functions with the coefficients evaluated on the cluster, and calculate the
function in a two dimensional system within the $1/S$ expansion. Due to the
anisotropic terms, the spectral intensities are considerably enhanced around
momentum transfer $\textbf{q}=0$ in both one-magnon and two-magnon excitation
channels. This finding may be experimentally confirmed by examining carefully
the $\textbf{q}$-dependence of the spectra.
