On irreducibility of modules of Whittaker type: Twisted modules and nonabelian orbifolds

In [1], we extended the Dong-Mason theorem on irreducibility of modules for cyclic orbifold vertex algebras (cf. [12]) to the entire category of weak modules and applied this result to Whittaker modules. In this paper, we present further generalizations of these results for nonabelian orbifolds of vertex operator superalgebras. Let V be a vertex superalgebra of a countable dimension and let G be a finite subgroup of $\mathrm{Aut}\left(V\right)$. Assume that $h\in Z\left(G\right)$ where $Z\left(G\right)$ is the center of the group G. For any irreducible h–twisted (weak) V–module M, we prove that if $M\ncong g\circ M$ for all $g\in G$ then M is also irreducible as $V^G$–module. We also apply this result to examples and give irreducibility of modules of Whittaker type for orbifolds of Neveu-Schwarz vertex superalgebras, Heisenberg vertex algebras, Virasoro vertex operator algebra and Heisenberg-Virasoro vertex algebra.