Grupe_2007_AJ_133_2216.pdf (133.92 kB)
Redshift filtering by Swift apparent X-ray column density
journal contributionposted on 2012-10-24, 08:56 authored by D. Grupe, J. A. Nousek, D. E. vanden Berk, P. W. A. Roming, D. N. Burrows, O. Godet, J. Osborne, N. Gehrels
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.