Blackbody components in gamma-ray bursts spectra?

We study seven gamma-ray bursts (GRBs), detected both by the Burst And Transient Source Experiment (BATSE) instrument, onboard the Compton Gamma-ray Observatory, and by the Wide Field Camera (WFC), onboard BeppoSAX. These bursts have measured spectroscopic redshifts and are a sizeable fraction of the bursts defining the correlation between the peak energy E-peak (i.e. the peak of the vF(v) spectrum) and the total prompt isotropic energy E-iso, (so-called 'Amati' relation). Recent theoretical interpretations of this correlation assume that blackbody emission dominates the time-resolved spectra of GRBs, even if, in the time integrated spectrum, its presence may be hidden by the change of its temperature and by the dilution of a possible non-thermal power-law component. We perform a time-resolved spectral analysis and show that the sum of a power law and a blackbody gives acceptable fits to the time-dependent spectra within the BATSE energy range but overpredicts the flux in the WFC X-ray range. Moreover, a fit with a cut-off power law plus a blackbody is consistent with the WFC data but the blackbody component contributes a negligible fraction of the total flux. On the contrary, we find that fitting the spectra with a Band model or a simple cut-off power-law model yields an X-ray flux and spectral slope which well matches the WFC spectra.