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 BAT and XRT data. The observed spectra 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 hundred seconds in the prompt tail. This is one of the steepest X-ray spectra ever observed, making it quite difficult to explain by simple electron acceleration and synchrotron radiation. Then, we applied an alternative spectral fitting using a broken power-law with exponential cutoff (BPEC) model. It is valid to consider the situation that the cutoff energy is equivalent to the synchrotron frequency of the maximum energy electrons in their energy distribution. Since the spectral cutoff appears in the soft X-ray band, we conclude the electron acceleration has been inefficient in the internal shocks of GRB 060904A. These cutoff spectra suddenly disappeared at the transition time from the prompt tail phase to the shallow decay one. After that, typical afterglow spectra with the photon indices of 2.0 are continuously and preciously monitored by both XRT and Suzaku/XIS up to 1 day since the burst trigger time. We could successfully trace the temporal history of two characteristic break energies (peak energy and cutoff energy) and they show the time dependence of $\propto t^{-3} \sim t^{-4}$ while the following afterglow spectra are quite stable. This fact indicates that the emitting material of prompt tail is due to completely different dynamics from the shallow decay component. Therefore we conclude the emission sites of two distinct phenomena obviously differ from each other.Comment: 19 pages, 9 figures, accepted for publication in PASJ (Suzaku 2nd Special Issue

Spectral evolution of GRB 060904A observed with Swift and Suzaku

Abstract We observed an X-ray afterglow of GRB 060904A with the Swift and Suzaku satellites. We found a rapid spectral softening in the prompt tail observed by Swift. Adopting the absorbed power-law model to the observed spectra, the photon indices rapidly change from Γ = 1.2 ± 0.1 to Γ = 5.3 +0.7 −0.6 . It is quite difficult to explain this ultra soft spectrum by the simple electron acceleration and the synchrotron radiation. 1 Then we try to advanced spectral fitting with the broken power-law with exponential cutoff (BPEC) model, and the power-law with blackbody as seen in GRB 060218. The observed spectra can be explained by the both models. After the transition to the shallow decay phase, the spectral photon indices become steady as the typical value of Γ ∼ 2, and this value keeps flat toward the end of Suzaku observation. Through these observation, we confirmed that no significant spectral evolution was found during the shallow-classical-jet break phases