  We calculate the critical exponents of the threshold singularity for the
spectral density of the XXZ- spin chain at zero magnetic field for the lower
threshold. We show that the corresponding phase shifts are momentum-independent
and coinside with the predictions of the effective mobile impurity Hamiltonian
approach. We show that for the eigenstates with the high-energy particle with
the momentum $k$ which is much larger than the momenta of the particle-hole
excitations the formfactor is not described in the framework of the Luttinger
model even in the limit $k<<p_F$ but should be evaluated in the framework of
the effective mobile impurity Hamiltonian approach.
