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

We perform a time-resolved spectral analysis of bright, long Gamma-ray burst GRB 061007 using Suzaku/WAM and Swift/BAT. Thanks to the large effective area of the WAM, we can investigate the time evolution of the spectral peak energy, E peak and the luminosity L t iso with 1-sec time resolution, and we find that the time-resolved pulses also satisfy the Epeak −Liso 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 peak value by about factor 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 the fireball dynamics in real time.

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