Automorphisms of (affine) SL(2,q)-unitals

Abstract

SL$((2,q)$-unitals are unitals of order $q$ admitting a regular action of SL$((2,q)$ on the complement of some block. They can be obtained from affine SL$((2,q)$-unitals via parallelisms. We compute a sharp upper bound for automorphism groups of affine SL$((2,q)$-unitals and show that exactly two parallelisms are fixed by all automorphisms. In nonclassical SL$((2,q)$-unitals obtained as closures of affine SL$((2,q)$-unitals via those two parallelisms, we show that there is one block fixed under the full automorphism group.