Spectral evolution of GRB 060904A observed with Swift and Suzaku

Abstract

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