2011A&A...526A..49G

The emission process responsible for the so-called ``prompt'' emission of gamma-ray bursts is still unknown. A number of empirical models fitting the typical spectrum still lack a satisfactory interpretation. A few GRB spectral catalogues derived from past and present experiments are known in the literature and allow to tackle the issue of spectral properties of gamma-ray bursts on a statistical ground. We extracted and studied the time-integrated photon spectra of the 200 brightest GRBs observed with the Gamma-Ray Burst Monitor which flew aboard the BeppoSAX mission (1996-2002) to provide an independent statistical characterisation of GRB spectra. The spectra have a time-resolution of 128 s and consist of 240 energy channels covering the 40-700 keV energy band. The 200 brightest GRBs were selected from the complete catalogue of 1082 GRBs detected with the GRBM (Frontera et al., 2009ApJS..180..192F), whose products are publicly available and can be browsed/retrieved using a dedicated web interface. The spectra were fit with three models: a simple power law, a cut-off power law or a Band model. We derived the sample distributions of the best-fitting spectral parameters and investigated possible correlations between them. For a few, typically very long GRBs, we also provide a loose (128-s) time-resolved spectroscopic analysis. The typical photon spectrum of a bright GRB consists of a low-energy index around 1.0 and a peak energy of the νF_ν spectrum E_p≃240keV in agreement with previous results on a sample of bright CGRO/BATSE bursts. Spectra of ∼35% of GRBs can be fit with a power law with a photon index around 2, indicative of peak energies either close to or outside the GRBM energy boundaries. We confirm the correlation between E_p and fluence, in agreement with previous results, with a logarithmic dispersion of 0.13 around the power law with index 0.21±0.06. This is shallower than its analogous in the GRB rest-frame, the Amati relation, between the intrinsic peak energy and the isotropic-equivalent released energy (slope of ∼0.5). The reason for this difference mainly lies in the instrumental selection effect connected with the finite energy range of the GRBM particularly at low energies. We confirm the statistical properties of the low-energy index and peak energy distributions found by other experiments. These properties are not yet systematically explained in the current literature with the proposed emission processes. The capability of measuring time-resolved spectra over a broadband energy range, ensuring precise measurements of parameters such as E_p, will be of key importance for future experiments.