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We observed an X-ray afterglow of GRB 060904A with the Swift and Suzakusatellites. We found rapid spectral softening during both the prompt tail phaseand the decline phase of an X-ray flare in the BAT and XRT data. The observedspectra were fit by power-law photon indices which rapidly changed from $\Gamma= 1.51^{+0.04}_{-0.03}$ to $\Gamma = 5.30^{+0.69}_{-0.59}$ within a few hundredseconds in the prompt tail. This is one of the steepest X-ray spectra everobserved, making it quite difficult to explain by simple electron accelerationand synchrotron radiation. Then, we applied an alternative spectral fittingusing a broken power-law with exponential cutoff (BPEC) model. It is valid toconsider the situation that the cutoff energy is equivalent to the synchrotronfrequency of the maximum energy electrons in their energy distribution. Sincethe spectral cutoff appears in the soft X-ray band, we conclude the electronacceleration has been inefficient in the internal shocks of GRB 060904A. Thesecutoff spectra suddenly disappeared at the transition time from the prompt tailphase to the shallow decay one. After that, typical afterglow spectra with thephoton indices of 2.0 are continuously and preciously monitored by both XRT andSuzaku/XIS up to 1 day since the burst trigger time. We could successfullytrace the temporal history of two characteristic break energies (peak energyand cutoff energy) and they show the time dependence of $\propto t^{-3} \simt^{-4}$ while the following afterglow spectra are quite stable. This factindicates that the emitting material of prompt tail is due to completelydifferent dynamics from the shallow decay component. Therefore we conclude theemission sites of two distinct phenomena obviously differ from each other.

Spectral evolution of GRB 060904A observed with Swift and Suzaku -- Possibility of Inefficient Electron Acceleration