Generalized Nikulin surfaces and irreducible symplectic fourfolds

A Nikulin surface is the minimal resolution of the quotient of a $K3$ surface $S$ by a symplectic involution $\iota_S$.

Equivalently, it is the $2$-dimensional component of the fixed locus of the involution induced by $\iota_S$ on the Hilbert scheme $S^{[2]}$.

We study $K3$ surfaces $F$ that are the $2$-dimensional component of the fixed locus of a symplectic involution $\iota$ on hyper-Kähler manifolds $X$ of $K3^{[2]}$-type; we call them generalized Nikulin surfaces.

We show that a projective $K3$ surface is a generalized Nikulin surface if and only if its Néron-Severi lattice contains primitively the lattice $E_7(-2)$.

Moreover, we show that the transcendental lattices $T_F$ and $T_{\widetilde{X/ \iota}}$, where $\widetilde{X/ \iota}$ is the terminalization of the quotient $X/\iota$, are Hodge isometric.

Finally, we describe projective models of generalized Nikulin surfaces of small degrees.