Swift captures the spectrally evolving prompt emission of GRB 070616

TL;DR

This paper analyzes the long-duration prompt emission and afterglow of GRB 070616, revealing spectral evolution and emphasizing the importance of broadband, high-resolution observations to understand GRB emission mechanisms.

Contribution

It provides a detailed spectral and temporal analysis of GRB 070616's prompt emission, highlighting the role of spectral evolution and broadband data in understanding GRB physics.

Findings

Spectral peak energy shifts from hard to soft during prompt emission

High-energy light curve remains flat for several hundred seconds

Steep flux decline linked to spectral evolution and curvature effect

Abstract

The origins of Gamma-ray Burst prompt emission are currently not well understood and in this context long, well-observed events are particularly important to study. We present the case of GRB 070616, analysing the exceptionally long-duration multipeaked prompt emission, and later afterglow, captured by all the instruments on-board Swift and by Suzaku WAM. The high energy light curve remained generally flat for several hundred seconds before going into a steep decline. Spectral evolution from hard to soft is clearly taking place throughout the prompt emission, beginning at 285 s after the trigger and extending to 1200 s. We track the movement of the spectral peak energy, whilst observing a softening of the low energy spectral slope. The steep decline in flux may be caused by a combination of this strong spectral evolution and the curvature effect. We investigate origins for the spectral evolution, ruling out a superposition of two power laws and considering instead an additional component dominant during the late prompt emission. We also discuss origins for the early optical emission and the physics of the afterglow. The case of GRB 070616 clearly demonstrates that both broadband coverage and good time resolution are crucial to pin down the origins of the complex prompt emission in GRBs.