Title: GRB120729A: external shock origin for both the prompt gamma-ray emission and afterglow Show Chinese title

Title: GRB120729A：瞬时伽马射线辐射和余辉的外部冲击起源

Abstract: Gamma-ray burst (GRB) 120729A was detected by Swift/BAT and Fermi/GBM, and then rapidly observed by Swift/XRT, Swift/UVOT, and ground-based telescopes. It had a single long and smooth \gamma-ray emission pulse, which extends continuously to the X-rays. We report Lick/KAIT observations of the source, and make temporal and spectral joint fits of the multiwavelength light curves of GRB 120729A. It exhibits achromatic light-curve behavior, consistent with the predictions of the external shock model. The light curves are decomposed into four typical phases: onset bump (Phase I), normal decay (Phase II), shallow decay (Phase III), and post-jet break (Phase IV). The spectral energy distribution (SED) evolves from prompt \gamma-ray emission to the afterglow with photon index from $\Gamma_{\rm \gamma}=1.36$ to $\Gamma \approx 1.75$. There is no obvious evolution of the SED during the afterglow. The multiwavelength light curves from \gamma-ray to optical can be well modeled with an external shock by considering energy injection, and a time-dependent microphysics model with $\epsilon_B\propto t^{\alpha_B}$ for the emission at early times, $T < T_{\rm 0} + 157$~s. Therefore, we conclude that both the prompt \gamma-ray emission and afterglow of GRB 120729A have the same external shock physical origin. Our model indicates that the $\epsilon_B$ evolution can be described as a broken power-law function with $\alpha_{\rm B,1} = 0.18 \pm 0.04$ and $\alpha_{\rm B,2} = 0.84 \pm 0.04$. We also systematically investigate single-pulse GRBs in the Swift era, finding that only a small fraction of GRBs (GRBs 120729A, 051111, and 070318) are likely to originate from an external shock for both the prompt \gamma-ray emission and afterglow. Show Chinese abstract

Abstract: 伽马射线暴（GRB）120729A被Swift/BAT和Fermi/GBM探测到，随后迅速由Swift/XRT、Swift/UVOT和地面望远镜进行观测。 它有一个单一的长而平滑的\gamma -射线辐射脉冲，持续延伸到X射线。 我们报告了Lick/KAIT对该源的观测，并对GRB 120729A的多波段光变曲线进行了时间和光谱联合拟合。 它表现出无色光变曲线行为，与外冲击模型的预测一致。 光变曲线被分解为四个典型阶段：初始峰（阶段I）、正常衰减（阶段II）、缓衰减（阶段III）和喷流后阶段（阶段IV）。 光谱能量分布（SED）从即时\gamma -射线辐射演化到余辉，光子指数从$\Gamma_{\rm \gamma}=1.36$到$\Gamma \approx 1.75$。 在余辉期间，SED没有明显的演化。 从\gamma -射线到光学的多波段光变曲线可以通过考虑能量注入的外部冲击模型，以及早期时间的时变微观物理模型，其中$\epsilon_B\propto t^{\alpha_B}$用于发射，时间为$T < T_{\rm 0} + 157$~s。 因此，我们得出结论，GRB 120729A的提示\gamma 射线辐射和余辉具有相同的外部冲击物理起源。 我们的模型表明，$\epsilon_B$演化可以描述为一个分段幂律函数，具有$\alpha_{\rm B,1} = 0.18 \pm 0.04$和$\alpha_{\rm B,2} = 0.84 \pm 0.04$。 我们还系统地研究了Swift时代的单脉冲伽马暴，发现只有少数伽马暴（GRB 120729A、051111和070318）可能同时起源于外部冲击，对于提示\gamma 射线辐射和余辉而言。