A Tale of Two Bursts

In the talk I will review our recent work on the most remarkable Gamma-Ray Bursts (GRBs) in the past two years: GRB 160625B and GRB 170817A. GRB 160625B was an extraordinarily bright burst with three isolated episodes separated by long quiescent intervals. By conducting detailed time-resolved spectral analysis in each episode, we found that the spectral properties of the first two sub-bursts are distinctly different, allowing us to for the first time observe a clear transition from thermal to non-thermal radiation between well-separated emission episodes within one single GRB. Such a transition is a strong indication of the change of jet composition from a fireball to a Poynting-flux-dominated jet. GRB 170817A was the first confirmed short GRB associated with gravitational event GW170817. By performing detailed analysis on the prompt emission data, we show that the fluence and spectral peak energy of this sGRB fall into the lower portion of the distributions of known sGRBs. Its peak isotropic luminosity is abnormally low. The estimated event rate density above this luminosity is at least ~190 Gpc^-3yr^-1, which is close to but still below the double neutron star merger event rate density. The physical implications of the data including the jet structure will also be discussed.

Dr Zhang Binbin (Nanjing University)
KIAA, 1st meeting room
Fri, 2018-03-30 12:00 to 13:00