  Strong observational evidence for a fluctuating ultraviolet background (UVB)
has been accumulating through a number of studies of the HI and HeII Lya forest
as well as accurate IGM metallicity measurements. UVB fluctuations could arise
both from the inhomogeneous distribution of the ionizing sources and/or from
radiative transfer (RT) through the filamentary IGM. In this study we
investigate, via numerical simulations, the role of RT effects such as
shadowing, self-shielding and filtering of the ionizing radiation, in giving
raise to a fluctuating UVB. We focus on possible detectable signatures of these
effects on quantities derived from Lya forest spectra, as photoionization rate
fluctuations, eta parameter (the HeII to HI column density ratio) distributions
and the IGM temperature at redshift about 3. We find that RT induces
fluctuations up to 60% in the UVB, which are tightly correlated to the density
field. The UVB mean intensity is progressively suppressed toward higher
densities and photon energies above 4 Ryd, due to the high HeII opacity.
Shielding of overdense regions (Delta > 5) from cosmic HeII ionizing radiation,
produces a decreaseing trend of eta with overdensity. Furthermore we find that
the mean eta value inferred from HI-HeII Lya forest observations can be
explained only by properly accounting for the actual IGM opacity. We outline
and discuss several implications of our findings.
