2024ApJ...969..162D

The spectra of high-redshift (z ≳ 6) quasars contain valuable information on the progression of the Epoch of Reionization. At redshifts z < 6, the observed Lyman-series forest shows that the intergalactic medium is nearly ionized, while at z > 7 the observed quasar damping wings indicate high neutral gas fractions. However, there remains a gap in neutral gas fraction constraints at 6 ≲ z ≲ 7 where the Lyman-series forest becomes saturated but damping wings have yet to fully emerge. In this work, we use a sample of 18 quasar spectra at redshifts 6.0 < z < 7.1 to close this gap. We apply neural networks to reconstruct the quasars' continuum emission around the partially absorbed Lyα line to normalize their spectra, and stack these continuum-normalized spectra in three redshift bins. To increase the robustness of our results, we compare the stacks to a grid of models from two hydrodynamical simulations, ATON and CROC, and we measure the volume-averaged neutral gas fraction, ${\bar{x}}_{\mathrm{HI}}$, while jointly fitting for the mean quasar lifetime, t_Q, for each stacked spectrum. We chronicle the evolution of neutral gas fraction using the ATON (CROC) models as follows: ${\bar{x}}_{\mathrm{HI}}={0.21}_{-0.07}^{+0.17}$ (${\bar{x}}_{\mathrm{HI}}={0.10}_{\lt {10}^{-4}}^{+0.73}$) at 〈z〉 = 6.10, ${\bar{x}}_{\mathrm{HI}}={0.21}_{-0.07}^{+0.33}$ (${\bar{x}}_{\mathrm{HI}}={0.57}_{-0.47}^{+0.26}$) at 〈z〉 = 6.46, and ${\bar{x}}_{\mathrm{HI}}={0.37}_{-0.17}^{+0.17}$ (${\bar{x}}_{\mathrm{HI}}={0.57}_{-0.21}^{+0.26}$) at 〈z〉 = 6.87. At the same time, we constrain the average quasar lifetime to be t_Q ≲ 7 Myr across all redshift bins, in good agreement with previous studies.