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We study quasar proximity zones in the redshift range $5.77 \leq z \leq 6.54$by homogeneously analyzing $34$ medium resolution spectra, encompassing botharchival and newly obtained data, and exploiting recently updated systemicredshift and magnitude measurements. Whereas previous studies found strongevolution of proximity zone sizes with redshift, and argued that this providesevidence for a rapidly evolving intergalactic medium (IGM) neutral fractionduring reionization, we measure a much shallower trend $\propto(1+z)^{-1.44}$.We compare our measured proximity zone sizes to predictions from hydrodynamicalsimulations post-processed with one-dimensional radiative transfer, and findgood agreement between observations and theory irrespective of the ionizationstate of the ambient IGM. This insensitivity to IGM ionization state has beenpreviously noted, and results from the fact that the definition of proximityzone size as the first drop of the smoothed quasar spectrum below the $10\%$flux transmission level probes locations where the ionizing radiation from thequasar is an order of magnitude larger than the expected ultraviolet ionizingbackground that sets the neutral fraction of the IGM. Our analysis alsouncovered three objects with exceptionally small proximity zones (two have $R_p< 1$proper Mpc), which constitute outliers from the observed distribution andare challenging to explain with our radiative transfer simulations. We considervarious explanations for their origin, such as strong absorption line systemsassociated with the quasar or patchy reionization, but find that the mostcompelling scenario is that these quasars have been shining for $\lesssim10^5$yr.

Implications of $z \sim 6$ Quasar Proximity Zones for the Epoch of Reionization and Quasar Lifetimes