Spectral evolution of GRB 060904A observed with Swift and Suzaku - Possibility of inefficient electron acceleration

We observed an X-ray afterglow of GRB 060904A with the Swift and Suzaku satellites. We found rapid spectral softening during both the prompt tail phase and the decline phase of an X-ray flare in the Swift/BAT and Swift/XRT data. The observed spectra were fit by power-law photon indices which rapidly changed from Γ = 1.51_-0.03^+0.04 to Γ = 5.30 _-0.59^+0.69 within a few hundred seconds. This is one of the steepest X-ray spectra ever observed, making it quite difficult to explain by simple electron-acceleration and synchrotron-radiation processes. We then applied an alternative broken power-law with exponential cutoff (BPEC) model. It is valid to consider the exponential shape is equivalent to a synchrotron cutoff. Since the spectral cutoff appears in the soft X-ray band, we conclude that the electron acceleration must be inefficient in the internal shocks of GRB 060904A. These cutoff spectra suddenly disappeared at the end of the prompt tail. After that, typical afterglow spectra with Γ = 2.0 have been continuously and preciously monitored by both Swift/XRT and Suzaku/XIS. We could successfully trace the temporal history of two characteristic break energies that have a time dependence of ∝t^-3-t^-4, while the following afterglow spectra are quite stable. This fact indicates that the emitting material of the prompt tail is due to completely different dynamics from the shallow decay component. Therefore, the emission sites of two distinct phenomena obviously differ from each other.