Redshift filtering by Swift apparent X-ray column density

Dirk Grupe, John A. Nousek, Daniel E. Vanden Berk, Peter W.A. Roming, David N. Burrows, Olivier Godet, Julian Osborne, Neil Gehrels

Research output: Contribution to journalArticlepeer-review

42 Scopus citations


We remark on the 8tility of an observational relation between the absorption column density in excess of the Galactic absorption column density, ΔNH = NH,fit - NH,gal, and redshift, z, determined from all 55 Swift-observed long bursts with spectroscopic redshifts as of 2006 December. The absorption column densities, NH,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 ΔNH > 2 × 1021 cm -2 are only present in bursts with redshifts z < 2. Low absorption column densities with ΔNH < 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.

Original languageEnglish (US)
Pages (from-to)2216-2221
Number of pages6
JournalAstronomical Journal
Issue number5
StatePublished - May 2007

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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