Computer Science
Scientific paper
Apr 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011adspr..47.1387f&link_type=abstract
Advances in Space Research, Volume 47, Issue 8, p. 1387-1403.
Computer Science
1
Scientific paper
We have analysed a sample of 328 time-integrated GRB prompt emission spectra taken via the Konus instrument on board the US GGS-Wind spacecraft between 2002 and 2004 using a couple of two-components models, Cut-off Power Law (CPL) + Power Law (PL) and blackbody (BB) + PL. The spectra show clear deviation from the Band function. The PL term is interpreted as the low energy tail of a nonthermal emission mechanism. The distributions of corresponding index β give values β < -2/3 consistent with synchrotron and synchrotron self-Compton mechanisms. The distribution of low energy index α associated with the CPL term shows clear discordance with synchrotron models for 31.4% of the analysed GRBs with values exceeding that for the line of death, α = -2/3. Then, a set of nonthermal radiation mechanisms producing harder slopes, i.e., α > -2/3, are presented and discussed. For the remaining majority (68.6%) of GRBs with CPL index α < -2/3, we show that optically thin synchrotron produced by a power law electron distribution of type, N(γ) ˜ γ-p, γ1 < γ < γ2, for finite energy range (γ2 ≠ ∞) is a likely emission mechanism with α ˜-(p + 1)/2 in the frequency range ν1 ≪ ν ≪ ν2 (where ν2 = η2ν1 with η = γ2/γ1), such that for p > 1/3, one gets α < -2/3. We also show that corresponding spectra in terms of Fν and νFν functions are peaked around frequency ν2 instead of ν1, respectively for p < 1 and p < 3. Besides, thermal emission is examined taking a single Planck function for fitting the low energy range. It can be interpreted as an early emission from the GRB fireball photosphere with observed mean temperature, kT′ ˜ 16.8 keV. Furthermore, we have performed a statistical comparison between the CPL + PL and BB + PL models finding comparable χ2-values for an important fraction of GRBs, which makes it difficult to distinguish which model and specific radiation mechanism (possible thermal or nonthermal γ-ray emissions) are best suitable for describing the reported data. Therefore, additional information for those bursts, such as γ-ray polarization, would be highly desirable in future determinations of GRBs observational data.
Fouka M.
Ouichaoui S.
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