Redshift Filtering by Swift Apparent X-Ray Column Density
Abstract
We remark on the utility 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 ΔNH 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.
- Publication:
-
The Astronomical Journal
- Pub Date:
- May 2007
- DOI:
- 10.1086/513014
- arXiv:
- arXiv:astro-ph/0612104
- Bibcode:
- 2007AJ....133.2216G
- Keywords:
-
- distance scale;
- gamma rays: bursts;
- Astrophysics
- E-Print:
- revised version including updates and the referee's comments, accepted for publication in the Astronomical Journal, 12 pages, 2 figures, 2 tables - v3 contains an update on the reference list