Campana, S. 2011 Swift暴通过X射线吸收得到的H的柱密度

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包含大量的暴的N_H

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Title:		The X-ray absorbing column density of a complete sample of bright Swift Gamma-Ray Bursts
Authors:		Campana, S.; Salvaterra, R.; Melandri, A.; Vergani, S. D.; Covino, S.; D'Avanzo, P.; Fugazza, D.; Ghisellini, G.; Sbarufatti, B.;Tagliaferri, G.
Publication:		eprint arXiv:1112.5111
Publication Date:		12/2011
Origin:		ARXIV
Keywords:		Astrophysics - High Energy Astrophysical Phenomena
Comment:		MNRAS in press, 6 pages, 3 figures, 1 table
Bibliographic Code:		2011arXiv1112.5111C

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.

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