  Magnetic ions such as Fe, Mn and Co with localized spins may be adsorbed on
the surface of topological insulators like \Bi. They form scattering centers
for the helical surface states which have a Dirac cone dispersion as long as
the local spins are disordered. However, the local density of states (LDOS) may
be severely modified by the formation of bound states. Commonly only elastic
scattering due to normal and exchange potentials of the adatom is assumed.
Magnetization measurements show, however, that considerable magnetic single ion
anisotropies exist which lead to a splitting of the local impurity spin states
resulting in a singlet ground state. Therefore inelastic scattering processes
of helical Dirac electrons become possible as described by a dynamical local
self energy of second order in the exchange interaction. The self energy
influences bound state formation and leads to significant new anomalies in the
LDOS at low energies and low temperatures which we calculate within T-matrix
approach. We propose that they may be used for spectroscopy of local impurity
spin states by appropriate tuning of chemical potential and magnetic field.
