Krongold 2013 X射线和光学所推出的介质柱密度不同的原因

主要内容:
X射线的比光学的高出10倍。（是不是如果没电离，对光学来说就是透明的？）

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Title:		An Explanation for the Different X-Ray to Optical Column Densities in the Environments of Gamma Ray Bursts: A Progenitor Embedded in a Dense Medium
Authors:		Krongold, Yair; Prochaska, J. Xavier
Affiliation:		AA(Instituto de Astronomia, Universidad Nacional Autonoma de Mexico, Apartado Postal 70-264, 04510 Mexico DF, Mexico xavier@ucolick.org), AB(Department of Astronomy and Astrophysics, UCO/Lick Observatory, University of California, 1156 High Street, Santa Cruz, CA 95064, USA)
Publication:		The Astrophysical Journal, Volume 774, Issue 2, article id. 115, 11 pp. (2013). (ApJ Homepage)
Publication Date:		09/2013
Origin:		IOP
Astronomy Keywords:		galaxies: ISM, gamma-ray burst: general
DOI:		10.1088/0004-637X/774/2/115
Bibliographic Code:		2013ApJ...774..115K

Abstract

We study the >~ 10 ratios in the X-ray to optical column densities inferred from afterglow spectra of gamma ray bursts (GRBs) due to gas surrounding their progenitors. We present time-evolving photoionization calculations for these afterglows and explore different conditions of their environment. We find that homogenous models of the environment (constant density) predict X-ray columns similar to those found in the optical spectra, with the bulk of the opacity being produced by neutral material at large distances from the burst. This result is independent of gas density or metallicity. Only models assuming a progenitor immersed in a dense (~10^2-4 cm^-3) cloud of gas (with radius ~10 pc), with a strong, declining gradient of density for the surrounding interstellar medium (ISM) are able to account for the large X-ray to optical column density ratios. However, to avoid an unphysical correlation between the size of this cloud and the size of the ionization front produced by the GRB, the models also require that the circumburst medium is already ionized prior to the burst. The inferred cloud masses are <~ 10⁶ M _⊙, even if low metallicities in the medium are assumed (Z ~ 0.1 Z _⊙). These cloud properties are consistent with those found in giant molecular clouds and our results support a scenario in which the progenitors reside within intense star formation regions of galaxies. Finally, we show that modeling over large samples of GRB afterglows may offer strong constraints on the range of properties in these clouds, and the host galaxy ISM.

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