  Using polarization-resolved resonant Raman spectroscopy, we explore
collective spin excitations of the chiral surface states in a three dimensional
topological insulator, Bi$_2$Se$_3$. We observe a sharp peak at 150 meV in the
pseudovector $A_2$ symmetry channel of the Raman spectra. By comparing the data
with calculations, we identify this peak as the transverse collective spin mode
of surface Dirac fermions. This mode, unlike a Dirac plasmon or a surface
plasmon in the charge sector of excitations, is analogous to a spin wave in a
partially polarized Fermi liquid, with spin-orbit coupling playing the role of
an effective magnetic field.
