http://arxiv.org/abs/1810.03129
The composition of ejecta is still an unsolved question in gamma-ray bursts (GRBs). Some previous studies have been suggested that the prompt emission in GRBs consists of several components giving rise to the observed spectral shape. Here, we present a systematic analysis to search an evidence of the transition from fireball to Poynting-flux-dominated outflow of GRBs, detected by the Gamma-ray Burst Monitor onboard the \emph{Fermi} satellite. We obtain 43 GRBs with multi-pulse prompt emission light curves, and 9/43 (21\%) bursts exhibit such a clear transition between different pulses within a single burst based on the time-integrated spectra. We further select 4 bursts with adequate data to perform a detail time-resolved spectral analysis and model discussion, and find 3 bursts show an early thermal precursor before the main bursts in both their time-integrated and time-resolved spectra. We then derived the outflow properties for those pulses which are involved in a thermal component, and find they are consistent with the typically observed properties of the photosphere emission. The rest pulses can be well explained by the optically-thin synchrotron emission component. Our analysis indicates that a good fraction of the multi-pulse {\it Fermi} bursts presents an observed evidence that the transition from fireball to Poynting-Flux-Dominated outflow.
L. Li
Tue, 9 Oct 18
49/77
Comments: 64 pages, 63 figures, 8 Tables. Submitted to ApJS. Comments welcome
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