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[SUBSCRIPT H] = N[SUBSCRIPT H, fit] - N[SUBSCRIPT 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[SUBSCRIPT 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 Delta N[SUBSCRIPT H] > 2 x 10^21 cm^- 2 are only present in bursts with redshifts z < 2. Low absorption column densities with Delta N[SUBSCRIPT H] < 1 x 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 X-Ray Telescope observable energy range for high-redshift bursts. We find a clear anticorrelation between Delta N[SUBSCRIPT 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.