2006A&A...455...91F

We present the far-UV spectrum of the quasar HS 1700+6416 taken with FUSE. This QSO provides the second line of sight with the HeII absorption resolved into a Lyα forest structure. Since HS 1700+6416 is slightly less redshifted (z_em=2.72) than HE 2347-4342, we only probe the post-reionization phase of HeII, seen in the evolution of the HeII opacity, which is consistent with a simple power law. The HeII/HI ratio η is estimated using a line profile-fitting procedure and an apparent optical depth approach, respectively. The expected metal line absorption in the far-UV is taken into account as well as molecular absorption of galactic H₂. About 27% of the η values are affected by metal line absorption. In order to investigate the applicability of the analysis methods, we create simple artificial spectra based on the statistical properties of the HI Lyα forest. The analysis of the artificial data demonstrates that the apparent optical depth method as well as the line profile-fitting procedure lead to confident results for restricted data samples only (0.01≤τ_HI≤0.1 and 12.0≤logN_HI≤13.0, respectively). The reasons are saturation in the case of the apparent optical depth and thermal line widths in the case of the profile fits. Furthermore, applying the methods to the unrestricted data set may mimic a correlation between the HeII/HI ratio and the strength of the HI absorption. For the restricted data samples a scatter of 10-15% in η would be expected even if the underlying value is constant. The observed scatter is significantly larger than expected, indicating that the intergalactic radiation background is indeed fluctuating. In the redshift range 2.58<z<2.72, where the data quality is best, we find η∼100, suggesting a contribution of soft sources like galaxies to the UV background.