摘要：We present time-resolved spectral analysis of prompt emission from GRB 160625B, one of the brightest bursts ever detected byFermiin its nine years of operations. Standard empirical functions fail to provide an acceptable fit to the GBM spectral data, which instead require the addition of a low-energy break to the fitting function. We introduce a new fitting function, called 2SBPL, consisting of three smoothly connected power laws. Fitting this model to the data, the goodness of the fits significantly improves and the spectral parameters are well constrained. We also test a spectral model that combines non-thermal and thermal (black body) components, but find that the 2SBPL model is systematically favoured. The spectral evolution shows that the spectral break is located aroundEbreak~100 keV, while the usualνFνpeak energy featureEpeakevolves in the 0.5–6 MeV energy range. The slopes below and aboveEbreakare consistent with the values –0.67 and –1.5, respectively, expected from synchrotron emission produced by a relativistic electron population with a low-energy cut-off. IfEbreakis interpreted as the synchrotron cooling frequency, the implied magnetic field in the emitting region is ~10 Gauss, i.e. orders of magnitudes smaller than the value expected for a dissipation region located at ~1013−14cm from the central engine. The low ratio betweenEpeakandEbreakimplies that the radiative cooling is incomplete, contrary to what is expected in strongly magnetized and compact emitting regions.
关键词：Key wordsengamma-ray burst: generalradiation mechanisms: non-thermalgamma-ray burst: individual: GRB 160625B
