Graph $C^\ast$-algebras with a $T_1$ primitive ideal space

We give necessary and sufficient conditions which a graph should satisfy in order for its associated $C^\ast$-algebra to have a $T_1$ primitive ideal space. We give a description of which one-point sets in such a primitive ideal space are open, and use this to prove that any purely infinite graph $C^\ast$-algebra with a $T_1$ (in particular Hausdorff) primitive ideal space, is a $c_0$-direct sum of Kirchberg algebras. Moreover, we show that graph $C^\ast$-algebras with a $T_1$ primitive ideal space canonically may be given the structure of a $C(\widetilde{\mathbb N})$-algebra, and that isomorphisms of their $\widetilde{\mathbb N}$-filtered $K$-theory (without coefficients) lift to $E(\widetilde{\mathbb N})$-equivalences, as defined by Dadarlat and Meyer.