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We examine a large set of synthetic quasar spectra to realistically assess the potential of using the relative sizes of highly ionized near‐zones in the Lyα and Lyβ forest as a probe of the neutral hydrogen content of the intergalactic medium (IGM) at   z  &gt; 6 . The scatter in the relative near‐zone size distribution, induced by underlying fluctuations in the baryonic density field and the filtering of ionizing radiation, is considerable even for fixed assumptions about the IGM neutral fraction. As a consequence, the current observational data cannot distinguish between an IGM which is significantly neutral or one which is highly ionized just above   z = 6 . Under standard assumptions for quasar ages and ionizing luminosities, a future sample of several tens of high‐resolution Lyα and Lyβ near‐zone spectra should be capable of distinguishing between a volume‐weighted neutral hydrogen fraction in the IGM which is greater or less than 10 per cent.

A closer look at using quasar near‐zones as a probe of neutral hydrogen in the intergalactic medium