Chronicling the reionization history at $6\lesssim z \lesssim 7$ with emergent quasar damping wings

Abstract: The spectra of high-redshift ($z\gtrsim 6$) quasars contain valuable information on the progression of the Epoch of Reionization (EoR). At redshifts $z<6$, the observed Lyman-series forest shows that the intergalactic medium (IGM) 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 $6\lesssim z \lesssim 7$ 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 Lyman $\alpha$ 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, $\bar{x}{\rm HI}$, while jointly fitting for the mean quasar lifetime, $t{\rm Q}$, for each stacked spectrum. We chronicle the evolution of neutral gas fraction using the ATON (CROC) models as follows: $\bar{x}{\rm HI} = 0.21{-0.07}^{+0.17}$ ($\bar{x}{\rm HI} = 0.10{<10^{-4}}^{+0.73}$) at $\langle z \rangle =6.10$, $\bar{x}{\rm HI} = 0.21{-0.07}^{+0.33}$ ($\bar{x}{\rm HI} =0.57{-0.47}^{+0.26}$) at $\langle z \rangle =6.46$, and $\bar{x}{\rm HI} = 0.37{-0.17}^{+0.17}$ ($\bar{x}{\rm HI} =0.57{-0.21}^{+0.26}$) at $\langle z \rangle =6.87$. At the same time we constrain the average quasar lifetime to be $t_{\rm Q} \lesssim 7\ {\rm Myr}$ across all redshift bins, in good agreement with previous studies.