Redshift filtering by Swift apparent X-ray column density

We remark on the 8tility of an observational relation between the absorption column density in excess of the Galactic absorption column density, ΔN_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 power-law 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 ΔN_H > 2 × 10²¹ cm ^-2 are only present in bursts with redshifts z < 2. Low absorption column densities with ΔN_H < 1 × 10²¹ 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 X-Ray Telescope observable energy range for high-redshift bursts. We find a clear anticorrelation between ΔN _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/near-IR follow-up observations. Ground-based spectroscopic redshift measurements of as many bursts as possible are crucial for gamma-ray burst science.