Context :

Aims : To reconstruct the spectral shape of the quasar ionizing radiation in the extreme-UV range ( 1 Ryd \leq E < 10 Ryd ) from the analysis of narrow absorption lines ( NAL ) of the associated systems .

Methods : Computational technique for inverse spectroscopic problems – Monte Carlo Inversion augmented by procedure of the spectral shape recovering and modified to account for the incomplete coverage of the light source .

Results : The ionizing spectra responsible for the ionization structure of the NAL systems require an intensity depression at E > 4 Ryd which is attributed to the He ii Lyman continuum opacity ( \tau ^ { He { \scriptscriptstyle II } } _ { c } \sim 1 ) .
A most likely source of this opacity is a quasar accretion disk wind .
The corresponding column density of H i in the wind is estimated as a few times 10 ^ { 16 } cm ^ { -2 } .
This amount of neutral hydrogen should cause a weak continuum depression at \lambda \raise 1.29 pt \hbox { $ < $ \kern - 7.5 pt \raise - 4.73 pt \hbox { $ \sim$ } } 912 Å ( rest-frame ) , and a broad and shallow absorption in H i Ly- \alpha .
If metallicity of the wind is high enough , other resonance lines of O vi , Ne vi - Ne viii , etc .
are expected .
In the analyzed QSO spectra we do observe broad ( stretching over 1000s km s ^ { -1 } ) and shallow ( \tau \ll 1 ) absorption troughs of H i Ly- \alpha and O vi \lambda \lambda 1031 , 1037 as well as continuum depressions at \lambda \raise 1.29 pt \hbox { $ < $ \kern - 7.5 pt \raise - 4.73 pt \hbox { $ \sim$ } } 912 Å which correspond to N ( H i ) \sim 5 \times 10 ^ { 16 } cm ^ { -2 } .
Observational data available in both the UV and X-ray ranges suggest that at least \sim 50 % of the quasar radiation passes through the gas opaque in the He ii Lyman continuum .
This means that the outcoming ionizing spectrum should have a pronounced intensity break at E > 4 Ryd with a depth of this break depending on the angle to the rotational axis of the accretion disk ( the larger the angle the deeper the break ) .
The QSO spectra with a discontinuity at 4 Ryd can influence the rate of the He ii reionization in the intergalactic medium and partly explain inhomogeneous ( patchy ) ionization structure of the intergalactic He ii observed at z \sim 3 .

Conclusions :