GRB 140515A at z=6.33: Constraints on the End of Reionization From a
  Gamma-ray Burst in a Low Hydrogen Column Density Environment
  We present the discovery and subsequent spectroscopy with Gemini-North of the
optical afterglow of the Swift gamma-ray burst (GRB) 140515A. The spectrum
exhibits a well-detected continuum at wavelengths longer than 8915 Angs with a
steep decrement to zero flux blueward of 8910 Angs due to Ly-alpha absorption
at redshift z~6.33. Some transmission through the Lyman-alpha forest is present
at 5.2<z<5.733, but none is detected at higher redshift, consistent with
previous measurements from quasars and GRB 130606A. We model the red damping
wing of Lyman-alpha in three ways that provide equally good fits to the data:
(a) a single host galaxy absorber at z=6.327 with log(N_HI)=18.62+/-0.08; (b)
pure intergalactic medium (IGM) absorption from z=6.0 to z=6.328 with a
constant neutral hydrogen fraction of x_HI=0.056+0.011-0.027; and (c) a hybrid
model with a host absorber located within an ionized bubble of radius 10
comoving Mpc in an IGM with x_HI=0.12+/-0.05 (x_HI<0.21 at the 2-sigma level).
Regardless of the model, the sharpness of the dropoff in transmission is
inconsistent with a substantial neutral fraction in the IGM at this redshift.
No narrow absorption lines from the host galaxy are detected, indicating a host
metallicity of [Z/H]<~ -0.8. Even if we assume that all of the hydrogen
absorption is due to the host galaxy, the column is unusually low for a GRB
sightline, similar to two out of the other three highest-redshift bursts with
measured log(N_HI). This is possible evidence that the escape fraction of
ionizing photons from normal star-forming galaxies increases at z>~6.
