We remark on the utility of an observational relation between the absorption
column density in excess of the Galactic absorption column density, ΞNHβ=NH,fitββNH,galβ, and redshift, z, determined from
all 55 Swift-observed long bursts with spectroscopic redshifts as of 2006
December. The absorption column densities, NH,fitβ, are determined
from powerlaw fits to the X-ray spectra with the absorption column density left
as a free parameter. We find that higher excess absorption column densities
with ΞNHβ>2Γ1021 cmβ2 are only present in bursts
with redshifts z<2. Low absorption column densities with ΞNHβ<1Γ1021 cmβ2 appear preferentially in high-redshift bursts. Our
interpretation is that this relation between redshift and excess column density
is an observational effect resulting from the shift of the source rest-frame
energy range below 1 keV out of the XRT observable energy range for high
redshift bursts. We found a clear anti-correlation between ΞNHβ
and z that can be used to estimate the range of the maximum redshift of an
afterglow. A critical application of our finding is that rapid X-ray
observations can be used to optimize the instrumentation used for ground-based
optical/NIR follow-up observations. Ground-based spectroscopic redshift
measurements of as many bursts as possible are crucial for GRB science.Comment: revised version including updates and the referee's comments,
accepted for publication in the Astronomical Journal, 12 pages, 2 figures, 2
tables - v3 contains an update on the reference lis