The Opacity of the Intergalactic Medium Measured Along Quasar Sightlines
  at $z\sim 6$
  We publicly release a new sample of $34$ medium resolution quasar spectra at
$5.77\leq z_{\rm em}\leq6.54$ observed with the Echellette Spectrograph and
Imager (ESI) on the Keck telescope. This quasar sample represents an ideal
laboratory to study the intergalactic medium (IGM) during the end stages of the
epoch of reionization, and constrain the timing and morphology of the phase
transition. For a subset of $23$ of our highest signal-to-noise ratio spectra
(S/N$>7$, per $10\,{\rm km\,s^{-1}}$ pixel), we present a new measurement of
the Lyman-$\alpha$ (Ly$\alpha$) forest opacity spanning the redshift range
$4.8\lesssim z\lesssim6.3$. We carefully eliminate spectral regions that could
be causing biases in our measurements due to additional transmitted flux in the
proximity zone of the quasars, or extra absorption caused by strong intervening
absorption systems along the line of sight. We compare the observed evolution
of the IGM opacity with redshift to predictions from a hydrodynamical
simulation with uniform ultraviolet background (UVB) radiation, as well as two
semi-numerical patchy reionization models, one with a fluctuating UVB and
another with a fluctuating temperature field. Our measurements show a steep
rise in opacity at $z\gtrsim5.0$ and an increased scatter and thus support the
picture of a spatially inhomogeneous reionization process, consistent with
previous work. However, we measure significantly higher optical depths at
$5.3\lesssim z\lesssim5.7$ than previous studies, which reduces the contrast
between the highest opacity Gunn-Peterson troughs and the average opacity trend
of the IGM, which may relieve some of the previously noted tension between
these measurements and reionization models.
