Analysis of GRB 080319B and GRB 050904 within the fireshell model: Evidence for a broader spectral energy distribution

Abstract

The observation of GRB080319B, with an isotropic energy E iso = 1.32 × 1054 erg, and GRB050904, with E iso = 1.04 × 1054 erg, offers 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 1054 erg, well outside the energy range where it has been successfully tested up to now (1049-1053 erg). We find that in the low-energy region, the prompt emission spectra observed by Swift Burst Alert Telescope (BAT) reveals more power than theoretically predicted. The opportunities offered by these observations to improve the fireshell model are outlined in this paper. One of the distinguishing features of the fireshell model is that it relates the observed GRB 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 due to the discrepancy of the theoretical and observed light curves and spectra in the fireshell model for GRBs 080319B and 050904. A new phenomenological parameter α is correspondingly introduced in the model. We perform numerical simulations of the prompt emission in the Swift BAT bandpass by assuming different values of α within the fireshell model. 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 α = -1.8 for both of them leads 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. © © 2012. The American Astronomical Society. All rights reserved..