Analysis of GRB 080319B and GRB 050904 within the fireshell model:
  evidence for a broader spectral energy distribution
  (Shortened) GRB080319B, with an isotropic energy E_{iso}=1.32x10^{54}erg, and
GRB050904, with E_{iso}=1.04x10^{54}erg, offer the possibility of studying the
spectral properties of the prompt radiation of two of the most energetic
Gamma-Ray Bursts (GRBs). This allows us to probe the validity of the fireshell
model for GRBs beyond 10^{54}erg, well outside the energy range where it has
been successfully tested up to now (10^{49}-10^{53}erg). We find that in the
low energy region, the prompt emission spectra observed by Swift BAT reveals
more power than theoretically predicted. The opportunities offered by these
observations to improve the fireshell model are outlined. One of the
distinguishing features of the fireshell model is that it relates the observed
spectra to the spectrum in the comoving frame of the fireshell. Originally, a
fully radiative condition and a comoving thermal spectrum were adopted. An
additional power-law in the comoving thermal spectrum is required [...] in the
fireshell model for GRBs 080319B and 050904. A new phenomenological parameter
\alpha is correspondingly introduced in the model. We perform numerical
simulations of the prompt emission in the Swift BAT bandpass by assuming
different values of \alpha [...]. We compare them with the GRB080319B and
GRB050904 observed time-resolved spectra, as well as with their time-integrated
spectra and light curves. Although GRB080319B and GRB050904 are at very
different redshifts (z=0.937 and z=6.29 respectively), a value of \alpha=-1.8
leads for both of them to a good agreement between the numerical simulations
and the observed BAT light curves, time-resolved and time-integrated spectra.
Such a modified spectrum is also consistent with the observations of previously
analyzed less energetic GRBs and reasons for this additional agreement are
given. Perspectives for future low energy missions are outlined.
