Positive spectral lags are commonly observed in gamma-ray burst (GRB) prompt phase where soft photons lag behind hard ones in their spectral studies. In contrast to this pattern, a fraction of GRBs show a negative spectral lag where hard photons arrive later compared to soft photons. Similarly, recent Fermi-Large Area Telescope observations show a late onset of high-energy photons in most GRB observations. A fraction of GRBs show a transition from positive to negative lags. Such negative lags and the spectral lag transition have no convincing explanation. We show that a structured GRB jet with velocity shear naturally produces both positive and negative spectral lags. The high-energy photons gain energy from repeated scattering with shearing layers and subsequently escape from higher altitudes. Hence, these photons are delayed compared to soft photons producing a negative spectral lag. The inner jet has no shear, and a positive lag appears to provide a unified picture of spectral lags in GRBs. The theory predicts a flip in spectral lag from positive to negative within the evolution of the prompt phase. Comparison of the observed lags with the prediction of the theory limits the possible range of GRB jet Lorentz factors to a few tens.

中文翻译：

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由于结构化射流的剪切压缩而导致的伽马射线暴瞬发相中负光谱滞后和正光谱滞后的统一理论


在伽马射线暴 （GRB） 瞬发阶段通常观察到正光谱滞后，其中软光子在其光谱研究中滞后于硬光子。与这种模式相反，一小部分 GRB 显示出负的光谱滞后，与软光子相比，硬光子到达的时间较晚。同样，最近的费米大面积望远镜观测表明，在大多数 GRB 观测中，高能光子的起始时间较晚。一小部分 GRB 显示从正滞后到负滞后的转变。这种负滞后和频谱滞后转换没有令人信服的解释。我们表明，具有速度剪切的结构化 GRB 射流自然会产生正和负光谱滞后。高能光子从剪切层的重复散射中获得能量，随后从更高的高度逸出。因此，与产生负光谱滞后的软光子相比，这些光子是延迟的。内部射流没有剪切，正滞后似乎提供了 GRB 中光谱滞后的统一画面。该理论预测，在提示阶段的演变中，频谱滞后会从正到负。将观测到的滞后与理论预测的比较将 GRB 喷流洛伦兹因子的可能范围限制在几十个。