We remark on the utility of an observational relation between the absorption column density in excess of the Galactic absorption column density , \Delta N _ { H } = N _ { H,fit } - N _ { H,gal } , and redshift , z , determined from all 55 Swift -observed long bursts with spectroscopic redshifts as of 2006 December .
The absorption column densities , N _ { H,fit } , 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 \Delta N _ { H } > 2 \times 10 ^ { 21 } cm ^ { -2 } are only present in bursts with redshifts z < 2 .
Low absorption column densities with \Delta N _ { H } < 1 \times 10 ^ { 21 } cm ^ { -2 } 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 \Delta N _ { H } 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 .