Spectral analysis of time-integrated Konus–Wind GRBs: Implication on radiative mechanisms

Abstract

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 , γ 1 < γ < γ 2, for finite energy range ( γ 2 ≠ ∞) is a likely emission mechanism with α ∼−( p + 1)/2 in the frequency range ν 1 ≪ ν ≪ ν 2 (where ν 2 = η 2 ν 1 with η = γ 2/ γ 1), such that for p > 1/3, one gets α < −2/3. We also show that corresponding spectra in terms of F ν and νF ν functions are peaked around frequency ν 2 instead of ν 1, 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 χ 2-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.