EDP Sciences
Free access
Volume 407, Number 2, August IV 2003
Page(s) 393 - 402
Section Cosmology
DOI http://dx.doi.org/10.1051/0004-6361:20030871

A&A 407, 393-402 (2003)
DOI: 10.1051/0004-6361:20030871

Emission and absorption lines of gamma-ray bursts affected by the relativistic motion of fireball ejecta

Yi-Ping Qin

National Astronomical Observatories/Yunnan Observatory, Chinese Academy of Sciences, PO Box 110, Kunming, Yunnan 650011, PR China
(Received 29 November 2002 / Accepted 12 May 2003)

We display by numerical calculation how rest frame spectral lines appear in the observed spectrum of gamma-ray bursts due to the Doppler effect in the fireball framework. The analysis shows that: a) in the spectrum of a relativistically expanding fireball, all rest frame lines would shift to higher energy bands and would be significantly smoothed; b) rest frame weak narrow emission lines as well as narrow absorption lines and absorption line forests would be smoothed and would hardly be detectable; c) the features of rest frame broad emission lines as well as both strong and weak broad absorption lines would remain almost unchanged and therefore would be easier to detect; d) deep gaps caused by rest frame broad absorption lines would be significantly filled; e) a rest frame emission line forest would form a single broad line feature; f) the observed relative width of the rest frame very narrow line would approach 0.162; g) when the Lorentz factor  $\Gamma $ is large enough, the observed line frequency $\nu _{\rm line}$ and the rest frame line frequency $\nu _{\rm0,\rm line} $ would be related by $\nu _{\rm line}\approx 2\Gamma \nu _{\rm0,line}$. We also investigate the effect of time dependence of the line intensity and the effect of variation of  $\Gamma $. We find that the feature of rest frame dimming narrow emission lines would disappear when  $\Gamma $ is very large. The form of emission lines would be sharp on both edges when $\Gamma $ varies with time. This phenomenon depends not only on the initial Lorentz factor but also on the observation time.

Key words: gamma-rays: bursts -- gamma-rays: theory -- radiation mechanisms: nonthermal -- relativity

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