Gamma-ray burst precursors as the remnant of the thermal radiation initially trapped in the fireball

In the standard fireball model of gamma-ray bursts (GRBs), the fireball starts with an optically thick phase. As it expands, the fireball becomes optically thin at some stage. The thermal radiation trapped in the originally opaque fireball then leaks out, producing a transient event. The appearance of the event is investigated in the framework of a homogeneous, spherically symmetric and freely expanding fireball produced instantly by an explosive process without continuous injection of mass and energy. We find that, generally, the event has a time duration shorter than that of the main burst, which is presumably produced by the internal shock after the fireball becomes optically thin. The event is separated from the main burst by a quiescent time interval, and is weaker than the main burst at least in a high-energy band. Hence, the event corresponds to a GRB precursor. The precursor event predicted by our model has a smooth and Fast Rise and Exponential Decay (FRED) shaped light curve, and a quasi-thermal spectrum. Typically, the characteristic blackbody photon energy is in the X-ray band. However, if the distortion of the blackbody spectrum by electron scattering is considered, the characteristic photon energy could be boosted to the gamma-ray band. Our model may explain a class of observed GRB precursors – those having smooth and FRED-shaped light curves and quasi-thermal spectra.     

火球框架下的不同角区域形成的脉冲

在火球模型下,研究了不同的无联系的和有联系的角区域的脉冲性质。发现:(1)FRED脉冲形状的上升和下降阶段来自于火球面的不同角区域,并且上升部分主要来自于靠近视线轴向的角区域部分;(2)整个火球形成的FRED脉冲形状主要是有张角为γ6/10的角区域部分提供;(3)球形火球或者张角大于γ6/10的喷流可以形成FRED脉冲形状。如果伽玛暴源不是球形火球或者也不是张角大于γ6/10的喷流,则形成的观测脉冲不是FRED形状。