Nuclear suppression factor for the $\gamma+{\rm Pb}\to {\rm J}/\psi+{\rm Pb}$ process as a function of $W_{\gamma\, \mathrm{Pb, n}}$ (lower axis) or Bjorken-$x$ (upper axis). The solid markers represent the measurement. The vertical line across a marker is the uncorrelated uncertainty. The height of an empty box is the sum in quadrature of the correlated systematic uncertainties and the effect of migrations across neutron classes. A gray box represents the theoretical uncertainty coming from the computation of the photon flux and of the impulse approximation. The lines depict the prediction of the different models discussed in Sec. 2. The open triangular and square markers show the nuclear suppression factor extracted in Refs. [17,18] using ALICE Run 1 data.