EDP Sciences
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Volume 393, Number 2, October II 2002
Page(s) 409 - 423
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20021038

A&A 393, 409-423 (2002)
DOI: 10.1051/0004-6361:20021038

Time resolved spectral analysis of bright gamma ray bursts

G. Ghirlanda1, A. Celotti1 and G. Ghisellini2

1  SISSA/ISAS, Trieste - V. Beirut 2-4, 34014 Trieste, Italy
2  Osservatorio Astronomico di Brera, via Bianchi 46, Merate (LC), Italy

(Received 15 January 2002 / Accepted 27 June 2002)

We present the time integrated and time resolved spectral analysis of a sample of bright bursts selected with $F_{\rm peak} \ge 20\
\mathrm{phot\ cm^{-2}\ s^{-1}}$ from the BATSE archive. We fitted four different spectral models to the pulse time integrated and time resolved spectra. We compare the low energy slope of the fitted spectra with the prediction of the synchrotron theory [ predicting photon spectra softer than $N(E)\propto E^{-2/3}$] , and test, through direct spectral fitting, the synchrotron shock model. We point out that differences in the parameters distribution can be ascribed to the different spectral shape of the models employed and that in most cases the spectrum can be described by a smoothly curved function. The synchrotron shock model does not give satisfactory fits to the time averaged and time resolved spectra. Finally, we derive that the synchrotron low energy limit is violated in a considerable number of spectra both during the rise and decay phase around the peak.

Key words: gamma rays: bursts -- methods: data analysis -- radiation mechanisms: non-thermal

Offprint request: G. Ghirlanda, ghirland@sissa.it

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