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We study the process of cosmic reionization and estimate the ionizingbackground in the IGM using the Lyman series absorption in the spectra of thefour quasars at 5.7<z<6.3 discovered by the SDSS. We derive the evolution ofthe ionizing background at high redshifts, using both semi-analytic techniquesand cosmological simulations to model the density fluctuations in the IGM. Theexistence of the complete Ly alpha Gunn-Peterson trough in the spectrum of thez=6.28 quasar SDSS 1030+0524 indicates a photoionization rate Gamma_{-12} atz~6 lower than 0.08, at least a factor of 6 smaller than the value at z~3. TheLy beta and Ly gamma Gunn-Peterson troughs give an even stronger limitGamma_{-12}<0.02 due to their smaller oscillator strengths, indicating that theionizing background in the IGM at z~6 is more than 20 times lower than that atz~3. Meanwhile, the volume-averaged neutral hydrogen fraction increases from10^{-5} at z~3 to >10^{-3} at z~6. At this redshift, the mass-averaged neutralhydrogen fraction is larger than 1%; the mildly overdense regions (delta > 3)are still mostly neutral and the comoving mean free path of ionizing photons isshorter than 8 Mpc. Comparison with simulations of cosmological reionizationshows that the observed properties of the IGM at z~6 are typical of those inthe era at the end of the overlap stage of reionization when the individual HIIregions merge. Thus, z~6 marks the end of the reionization epoch. The redshiftof reionization constrains the small scale power of the mass densityfluctuations and the star forming efficiency of the first generation ofobjects.Comment: AJ accepted, 27 pages; minor change

Evolution of the Ionizing Background and the Epoch of Reionization from the Spectra of [ITAL][CLC]z[/CLC][/ITAL] ∼ 6 Quasars