  The interaction of a strong flux of photons with matter through an
ionizing-front (I-front) is an ubiquitous phenomenon in the context of
astrophysics and inertial confinement fusion (ICF) where intense sources of
radiation put matter into motion. When the opacity of the irradiated material
varies continuously in the radiation spectral domain, only one single I-front
is formed. In contrast, as numerical simulations tend to show, when the opacity
of the irradiated material presents a sharp edge in the radiation spectral
domain, a second I-front (an edge-front) can form. A full description of the
mechanism behind the formation of this edge-front is presented in this article.
It allows to understand extra shocks (edge-shocks), displayed by ICF
simulations, that might affect the robustness of the design of fusion capsules
in actual experiments. Moreover, it may have consequences in various domains of
astrophysics where ablative flows occur.
