The X-ray absorbing column density of a complete sample of bright Swift Gamma-Ray Bursts

TL;DR

This study analyzes the intrinsic X-ray absorbing column densities of a complete, bright Swift GRB sample, revealing a mild redshift increase and a strong link between high column densities and dark, dust-rich environments.

Contribution

It provides the first detailed distribution of intrinsic X-ray column densities for a complete Swift GRB sample, highlighting their correlation with darkness and environmental properties.

Findings

Mean log(NH/cm-2)=21.7+-0.5

Mild increase of NH with redshift

Dark GRBs have higher X-ray column densities

Abstract

A complete sample of bright Swift Gamma-ray Bursts (GRBs) has been recently selected by Salvaterra et al. (2011). The sample has a high level of completeness in redshift (91%). We derive here the intrinsic absorbing X-ray column densities of these GRBs making use of the Swift X-ray Telescope data. This distribution has a mean value of log(NH/cm-2)=21.7+-0.5. This value is consistent with the distribution of the column densities derived from the total sample of GRBs with redshift. We find a mild increase of the intrinsic column density with redshift. This can be interpreted as due to the contribution of intervening systems along the line of sight. Making use of the spectral index connecting optical and X-ray fluxes at 11 hr (beta_OX), we investigate the relation of the intrinsic column density and the GRB `darkness'. We find that there is a very tight correlation between dark GRBs and high X-ray column densities. This clearly indicates that the dark GRBs are formed in a metal-rich environment where dust must be present.