  The 2d Heisenberg model --- or 2d O(3) model --- is popular in condensed
matter physics, and in particle physics as a toy model for QCD. Along with
other analogies, it shares with 4d Yang-Mills theories, and with QCD, the
property that the configurations are divided in topological sectors. In the
lattice regularisation the topological charge $Q$ can still be defined such
that $Q \in \mathbb{Z}$. It has generally been observed, however, that the
topological susceptibility $\chi_{\rm t} = \langle Q^2 \rangle / V$ does not
scale properly in the continuum limit, i.e. that the quantity $\chi_{\rm t}
\xi^2$ diverges for $\xi \to \infty$ (where $\xi$ is the correlation length in
lattice units). Here we address the question whether or not this divergence
persists after the application of the Gradient Flow.
