Xu, Wen 2010 用余辉的谱探测探测路上中性氢

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Title:		On the Formation of Lyα Emission from Resonantly Scattered Continuum Photons of Gamma-Ray Burst's Afterglow
Authors:		Xu, Wen; Wu, Xiang-Ping
Affiliation:		AA(National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China), AB(National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China)
Publication:		The Astrophysical Journal, Volume 710, Issue 2, pp. 1432-1443 (2010). (ApJ Homepage)
Publication Date:		02/2010
Origin:		IOP
ApJ Keywords:		cosmology: theory, gamma-ray burst: general, intergalactic medium, large-scale structure of universe
DOI:		10.1088/0004-637X/710/2/1432
Bibliographic Code:		2010ApJ...710.1432X

Abstract

The continuum spectrum of gamma-ray burst's (GRB) afterglow at Lyα wavelength is known to be otherwise featureless except for the existence of a pair of smooth damping wings. Resonant scattering of photons with the ambient neutral hydrogen around the GRB may alter this picture. We study the formation and evolution of the spectral imprint of these resonantly scattered photons in the context of GRB's afterglow. Based on an analytic model that includes photons that are scattered only once, as well as a complete treatment of all the scatterings using Monte Carlo simulations, we are able to calculate the spectrum and luminosity of this Lyα emission from a very early moment up to a late epoch. We find that the amount, the motion, and the geometry of the neutral hydrogen around the GRB, together with the time behavior of the source are the crucial factors that affect the predicted luminosity and spectral profile. The flux of the Lyα emission is found to be mainly contributed by photons that are scattered only once. The flux is of the order 10^-4-10^-9 relative to the undecayed maximum flux of the transmitted continuum, making the feature negligible but potentially observable. If not obscured by the host galaxy's damped Lyα absorption systems or intergalactic neutral hydrogen, the feature may appear sometime from 1 hr to several years when the directly transmitted light has faded away. This scattered emission feature can be distinguished from Lyα photons of other origins by its luminosity evolution and by its gradual narrowing of profile with time. The typical timescale for spectral variance is that of the light crossing time of a hydrogen clump close to the GRB. If observed, the resonant peaks' time-dependent behavior is a scanning probe on the distribution of neutral hydrogen in GRB's immediate neighborhood.

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