Interpretations of gamma-ray burst spectroscopy

Free access article

Issue		A&A Volume 429, Number 3, January III 2005


Page(s)		869 - 879
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20041364

A&A 429, 869-879 (2005)
DOI: 10.1051/0004-6361:20041364

I. Analytical and numerical study of spectral lags

F. Ryde

Stockholm Observatory, AlbaNova University Center, 106 91 Stockholm, Sweden
e-mail: felix@astro.su.se

(Received 28 May 2004 / Accepted 15 September 2004 )

Abstract
We describe the strong spectral evolution that occurs during a gamma-ray burst (GRB) pulse and the means by which it can be analyzed. In particular, we discuss the change of the light curve as a function of energy and the spectral lag. Based on observed empirical correlations, an analytical model is constructed which is used to describe the pulse shape and quantize the spectral lags and their dependences on the spectral evolution parameters. Using this model, we find that the spectral lag depends mainly on the pulse-decay time-scale and that hard spectra (with large spectral power-law indices $\alpha$ ) give the largest lags. Similarly, large initial peak-energies, $E_{\rm0}$ , lead to large lags, except in the case of very soft spectra. The hardness ratio is found to depend only weakly on $\alpha$ and the hardness-intensity-correlation index, $\eta$ . In particular, for low $E_{\rm0}$ , it is practically independent, and is determined mainly by $E_{\rm0}$ . The relation between the hardness ratio and the lags, for a certain $E_{\rm0}$ are described by power-laws, as $\alpha$ varies. These results are the consequences of the empirical description of the spectral evolution in pulses and can be used as a reference in analyses of observed pulses. We also discuss the expected signatures of a sample of hard spectral pulses (e.g. thermal or small pitch-angle synchrotron emission) versus soft spectral pulses (e.g. optically-thin synchrotron emission). Also the expected differences between a sample of low energetic bursts (such as X-ray flashes) and of high energetic bursts (classical bursts) are discussed.

Key words: gamma-rays: bursts -- methods: numerical

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