A second order expansion in the local limit theorem for a branching system of symmetric irreducible random walks

Consider a branching random walk, where the branching mechanism is governed by a Galton-Watson process, and the migration by a finite range symmetric irreducible random walk on the integer lattice ${\mathbb{Z}}^d$. Let $Z_n(z)$ be the number of the particles in the $n$-th generation at the point $z\in $\mathbb{Z}$^d$. Under the mild moment conditions for offspring distribution of the underlying Galton-Watson, we derive a second order expansion in the local limit theorem for $Z_n(z)$ for each given $z\in {\mathbb{Z}}^d$. That generalizes the results for simple branching random walks obtained by Gao [2018, SPA].