  Discovery of the cosmic reionization epoch would represent a significant
milestone in cosmology. We present Keck spectroscopy of the quasar SDSS
1044-0125, at z = 5.73. The spectrum shows a dramatic increase in the optical
depth at observed wavelengths lambda >~7550 A, corresponding to z_abs >~ 5.2.
Only a few small, narrow transmission regions are present in the spectrum
beyond that point, and out to the redshifts where the quasar signal begins. We
interpret this result as a signature of the trailing edge of the cosmic
reionization epoch, which we estimate to occur around <z> ~ 6 (as indeed
confirmed by subsequent observations by Becker et al.), and extending down to z
\~ 5.2. This behavior is expected in the modern theoretical models of the
reionization era, which predict a patchy and gradual onset of reionization. The
remaining transmission windows we see may correspond to the individual
reionization bubbles (Stromgren spheres) embedded in a still largely neutral
intergalactic medium, intersected by the line of sight to the quasar. Future
spectroscopic observations of quasars at comparable or larger redshifts will
provide a more detailed insight into the structure and extent of the
reionization era.
