  The forward Compton amplitude describes the process of virtual photon
scattering from a hadron and provides an essential ingredient for the
understanding of hadron structure. As a physical amplitude, the Compton tensor
naturally includes all target mass corrections and higher twist effects at a
fixed virtuality, $Q^2$. By making use of the second-order Feynman-Hellmann
theorem, the nucleon Compton tensor is calculated in lattice QCD at an
unphysical quark mass across a range of photon momenta $3 \lesssim Q^2 \lesssim
7$ GeV$^2$. This allows for the $Q^2$ dependence of the low moments of the
nucleon structure functions to be studied in a lattice calculation for the
first time. The results demonstrate that a systematic investigation of power
corrections and the approach to parton asymptotics is now within reach.
