Spin-0 fields and the NP-constants close to spatial infinity in Minkowski spacetime

The NP constants of massless spin-0 fields propagating in Minkowski spacetime are computed close to spatial and null infinity by means of Friedrich's \emph{$i^0$-cylinder}. Assuming certain regularity condition on the initial data ensuring that the field extends analytically to the critical sets, it is shown that the NP constants at future $\mathscr{I}^{+}$ and past null infinity $\mathscr{I}^{-}$ are independent of each other. In other words, the classical NP constants at $\mathscr{I}^{\pm}$ stem from different parts of the initial data given on a Cauchy hypersurface. In contrast, it is shown that, using a slight generalisation of the classical NP constants, the associated quantities ($i^0$-cylinder NP constants) do not require the regularity condition being satisfied and give rise to conserved quantities at $\mathscr{I}^{\pm}$ that are determined by the same piece of initial data which, in turn, correspond to the terms controlling the regularity of the field. Additionally, it is shown how the conservation laws associated to the NP constants can be exploited to construct, in flat space, heuristic asymptotic-system expansions which are sensitive to the logarithmic terms at the critical sets.