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 <~ 10
6 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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