Time-Evolution of Peak Energy and Luminosity Relation within Pulses for GRB 061007: Probing Fireball Dynamics

Abstract

We performed a time-resolved spectral analysis of a bright, long gamma-ray burst GRB 061007 using Suzaku/WAM and Swift/BAT. Because of the large effective area of the WAM, we can investigate the time evolution of the spectral peak energy E^t_peak and the luminosity L^t_iso with 1-s time resolution, and we find that the time-resolved pulses also satisfy the E_peak--L_iso relation, which was found for the time-averaged spectra of other bursts, suggesting the same physical conditions in each pulse. Furthermore, the initial rising phase of each pulse could be an outlier of this relation with higher E^t_peak value by a factor of 2. This difference could suggest that the fireball radius expands by a factor of 2--4 and/or bulk Lorentz factor of the fireball is decelerated by a factor of ∼4 during the initial phase, providing a new probe of fireball dynamics in real time.