  Recent experimental data on diffractive deep inelastic scattering collected
by the H1 and ZEUS Collaborations at HERA are analysed in a model with a
non-linear trajectory in the pomeron flux. The $t$ dependence of the
diffractive structure function $F_2^{D(4)}$ is predicted. The normalization of
the pomeron flux and the (weak) $Q^2$ dependence of the pomeron structure
function are revised as well.
