Multiple-component analysis of the time-resolved spectra of GRB 041006: A clue to the nature of the underlying soft component of GRBs

GRB 041006 was detected by HETE-2 on 2004 October 06. The light curves in four different energy bands display different features. At higher energy bands several peaks are seen in the light curve, while at lower energy bands a single broader bump dominates. It is expected that these different features are the result of a mixture of several components, each of which has different energetics and variability. We analyzed the time-resolved spectra, which were resolved into several components. These components can be classified into two distinct classes. One is a component that has an exponential decay of E _p with a characteristic timescale shorter than ∼ 30s; its spectrum is well represented by a broken power-law function, which is frequently observed in many prompt GRB emissions, so it should have an internal-shock origin. Another is a component whose E_p is almost unchanged with a characteristic timescale longer than ∼ 60s, and shows a very soft emission and slower variability. The spectrum is characterized by either a broken power law or a black-body spectrum. By assuming that the soft component is a thermal emission, the radiation radius is initially 4.4 x 10⁶km, which is a typical radius of a blue supergiant, and its expansion velocity is 2.4 x 10 ⁵km s^-1 in the source frame.