  In this Letter, we consider strong and electromagnetic (ultraperipheral)
mechanisms in proton-nucleus coherent diffraction at the CERN Large Hadron
Collider. We explicitly demonstrate the dominance of the latter and explain the
CMS data on the forward rapidity gap distribution in $pPb$ collisions at
$\sqrt{s_{NN}}=8.16$ TeV. In particular, we provide simple estimates, which
give a good, semi-quantitative description of both magnitude and shape of the
$\Delta \eta^F$ distribution in the Pomeron-proton topology. We also make
predictions for the proton-oxygen run.
