Redshift Filtering by Swift Apparent X-ray Column Density
  We remark on the utility of an observational relation between the absorption column density in excess of the Galactic absorption column density, $\Delta N_{\rm H} = N_{\rm H, fit} - N_{\rm H, gal}$, and redshift, z, determined from all 55 Swift-observed long bursts with spectroscopic redshifts as of 2006 December. The absorption column densities, $N_{\rm H, fit}$, 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 $\Delta N_{\rm H} > 2\times 10^{21}$ cm$^{-2}$ are only present in bursts with redshifts z$<$2. Low absorption column densities with $\Delta N_{\rm H} < 1\times 10^{21}$ 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 $\Delta N_{\rm H}$ 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. 
Dirk Grupe (PSU), John A. Nousek (PSU), Daniel E. vanden Berk (PSU),   Peter W.A. Roming (PSU), David N. Burrows (PSU), Oliv...