2008A&A...483...19L

We reconstruct the spectral shape of the quasar ionizing radiation in the extreme-UV range (1Ryd≤E<10Ryd) from the analysis of narrow absorption lines (NAL) of the associated systems. The computational technique for inverse spectroscopic problems - a Monte Carlo Inversion augmented by the spectral shape recovering procedure and modified to account for the incomplete coverage of the light source is used to analyze observations. The ionizing spectra responsible for the ionization structure of the NAL systems require an intensity depression at E>4Ryd which is attributed to the HeII Lyman continuum opacity (τ^HeII_c∼1). A most likely source of this opacity is a quasar accretion disk wind. The corresponding column density of HI in the wind is estimated as a few times 10¹⁶cm^–2. This amount of neutral hydrogen should cause a weak continuum depression at λ≲912Å (rest-frame), and a broad and shallow absorption in HI Ly-α. If the metallicity of the wind is high enough, other resonance lines of OVI, NeVI-NeVIII, etc. are expected. In the analyzed QSO spectra we do observe broad (stretching over 1000s.km/s) and shallow (τ≪1) absorption troughs of HI Ly-α and OVI λλ1031, 1037 as well as continuum depressions at λ≲912Å which correspond to N(HI)∼5x10¹⁶cm^–2. Observational data available in both the UV and X-ray ranges suggest that at least ∼50% of the quasar radiation passes through the gas opaque in the HeII Lyman continuum. This means that the outcoming ionizing spectrum should have a pronounced intensity break at E>4Ryd with the depth of this break depending on the angle with the rotational axis of the accretion disk (the larger the angle the deeper the break). The QSO spectra with a discontinuity at 4Ryd can influence the rate of HeII reionization in the intergalactic medium and partly explain the inhomogeneous (patchy) ionization structure of the intergalactic HeII observed at z∼3.