  The DIS diffractive cross section, $d\sigma^{diff}_{\gamma^* p \to XN}/dM_X$,
has been measured in the mass range $M_X < 15$ GeV for $\gamma^*p$ c.m.
energies $60 < W < 200$ GeV and photon virtualities $Q^2 = 7$ to 140 GeV$^2$.
For fixed $Q^2$ and $M_X$, the diffractive cross section rises rapidly with
$W$, $d\sigma^{diff}_{\gamma^*p \to XN}(M_X,W,Q^2)/dM_X \propto W^{a^{diff}}$
with $a^{diff} = 0.507 \pm 0.034 (stat)^{+0.155}_{-0.046}(syst)$ corresponding
to a $t$-averaged pomeron trajectory of $\bar{\alphapom} = 1.127 \pm 0.009
(stat)^{+0.039}_{-0.012} (syst)$ which is larger than $\bar{\alphapom}$
observed in hadron-hadron scattering. The $W$ dependence of the diffractive
cross section is found to be the same as that of the total cross section for
scattering of virtual photons on protons. The data are consistent with the
assumption that the diffractive structure function $F^{D(3)}_2$ factorizes
according to $\xpom F^{D(3)}_2 (\xpom,\beta,Q^2) = (x_0/ \xpom)^n
F^{D(2)}_2(\beta,Q^2)$. They are also consistent with QCD based models which
incorporate factorization breaking. The rise of $\xpom F^{D(3)}_2$ with
decreasing $\xpom$ and the weak dependence of $F^{D(2)}_2$ on $Q^2$ suggest a
substantial contribution from partonic interactions.
