  In this paper we present CRASH_alpha, the first radiative transfer code for
cosmological application that follows the parallel propagation of Ly_alpha and
ionizing photons. CRASH_alpha is a version of the continuum radiative transfer
code CRASH with a new algorithm to follow the propagation of Ly_alpha photons
through a gas configuration whose ionization structure is evolving. The
implementation introduces the time evolution for Ly_alpha photons (a feature
commonly neglected in line radiative transfer codes) and, to reduce the
computational time needed to follow each scattering, adopts a statistical
approach to the Ly_alpha treatment by making extensive use of pre-compiled
tables. With this statistical approach we experience a drastic increase of the
computational speed and, at the same time, an excellent agreement with the full
Ly_alpha radiative transfer computations of the code MCLy_alpha. We find that
the emerging spectra keep memory of the ionization history which generates a
given ionization configuration of the gas and, to properly account for this
effect, a self-consistent joint evolution of line and ionizing continuum
radiation as implemented in CRASH_alpha is necessary. A comparison between the
results from our code and from Ly_alpha scattering alone on a fixed HI density
field shows that the extent of the difference between the emerging spectra
depends on the particular configuration considered, but it can be substantial
and can thus affect the physical interpretation of the problem at hand. These
differences should furthermore be taken into account when computing the impact
of the Ly_alpha radiation on e.g. the observability of the 21 cm line from
neutral hydrogen at epochs preceeding complete reionization.
