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The spectra of high-redshift ($z\gtrsim 6$) quasars contain valuableinformation 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 wingsindicate high neutral gas fractions. However, there remains a gap in neutralgas fraction constraints at $6\lesssim z \lesssim 7$ where the Lyman seriesforest becomes saturated but damping wings have yet to fully emerge. In thiswork, we use a sample of 18 quasar spectra at redshifts $6.0<z<7.1$ to closethis gap. We apply neural networks to reconstruct the quasars' continuumemission around the partially absorbed Lyman $\alpha$ line to normalize theirspectra, and stack these continuum-normalized spectra in three redshift bins.To increase the robustness of our results, we compare the stacks to a grid ofmodels from two hydrodynamical simulations, ATON and CROC, and we measure thevolume-averaged neutral gas fraction, $\bar{x}_{\rm HI}$, while jointly fittingfor the mean quasar lifetime, $t_{\rm Q}$, for each stacked spectrum. Wechronicle the evolution of neutral gas fraction using the ATON (CROC) models asfollows: $\bar{x}_{\rm HI} = 0.21_{-0.07}^{+0.17}$ ($\bar{x}_{\rm HI} =0.10_{<10^{-4}}^{+0.73}$) 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 $\bar{x}_{\rm HI} = 0.37_{-0.17}^{+0.17}$ ($\bar{x}_{\rmHI} =0.57_{-0.21}^{+0.26}$) at $\langle z \rangle =6.87$. At the same time weconstrain the average quasar lifetime to be $t_{\rm Q} \lesssim 7\ {\rm Myr}$across all redshift bins, in good agreement with previous studies.

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