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We make use of hydrodynamical simulations of the intergalactic medium (IGM)to create model quasar absorption spectra. We compare these model spectra withthe observed Keck spectra of three z>6.2 quasars with full Gunn-Petersontroughs: SDSS J1148+5251 (z=6.42), SDSS J1030+0524 (z=6.28), and SDSSJ1623+3112 (z=6.22). We fit the probability density distributions (PDFs) of theobserved Ly alpha optical depths with those generated from the simulation, byexploring a range of values for the size of the quasar's surrounding HIIregion, R_S, the volume-weighted mean neutral hydrogen fraction in the ambientIGM, x_H, and the quasar's ionizing photon emissivity, N_Q. In order to avoidaveraging over possibly large sightline-to-sightline fluctuations in IGMproperties, we analyze each observed quasar independently. We find thefollowing results for J1148+5251, J1030+0524, and J1623+3112: The best-fitsizes R_S are 40, 41, and 29 (comoving) Mpc, respectively. These constraintsare tight, with only ~ 10% uncertainties, comparable to those caused byredshift--determination errors. The best-fit values of N_Q are 2.1, 1.3, and0.9 x 10^{57} s^{-1}, respectively, with a factor of ~ 2 uncertainty in eachcase. Finally, the best--fit values of x_H are 0.16, 1.0, and 1.0,respectively. The uncertainty in the case of J1148+5251 is large, and x_H isnot well constrained. However, for both J1030+0524 and J1623+3112, we find asignificant lower limit of x_H >~ 0.033. Our method is different from previousanalyses of the GP absorption spectra of these quasars, and our resultsstrengthen the evidence that the rapid end--stage of reionization is occurringnear z ~ 6.Comment: 12 pages, 4 figures; minor changes to match version accepted by the  Ap

Constraints on Reionization and Source Properties from the Absorption Spectra ofz> 6.2 Quasars