Topological Susceptibility of the 2d O(3) Model under Gradient Flow

The 2d O(3) model is widely used as a toy model for ferromagnetism and for Quantum Chromodynamics. With the latter it shares --- among other basic aspects --- the property that the continuum functional integral splits into topological sectors. Topology can also be defined in its lattice regularised version, but semi-classical arguments suggest that the topological susceptibility $\chi_{\rm t}$ does not scale towards a finite continuum limit. Previous numerical studies confirmed that the quantity $\chi_{\rm t}\, \xi^{2}$ diverges at large correlation length $\xi$. Here we investigate the question whether or not this divergence persists when the configurations are smoothened by the Gradient Flow (GF). The GF destroys part of the topological windings; on fine lattices this strongly reduces $\chi_{\rm t}$. However, even when the flow time is so long that the GF impact range --- or smoothing radius --- attains $\xi/2$, we do still not observe evidence of continuum scaling.