  We report on the nonequilibrium dynamics of the electronic structure of the
layered semiconductor Ta$_2$NiSe$_5$ investigated by time- and angle-resolved
photoelectron spectroscopy. We show that below the critical excitation density
of $F_{C} = 0.2$ mJ cm$^{-2}$, the band gap $narrows$ transiently, while it is
$enhanced$ above $F_{C}$. Hartree-Fock calculations reveal that this effect can
be explained by the presence of the low-temperature excitonic insulator phase
of Ta$_2$NiSe$_5$, whose order parameter is connected to the gap size. This
work demonstrates the ability to manipulate the band gap of Ta$_2$NiSe$_5$ with
light on the femtosecond time scale.
