标题： 在千兆电子伏特（$γ$）射线暴 GRB 221009A 中的明亮发射线 显示英文标题

标题： A bright megaelectronvolt emission line in $γ$-ray burst GRB 221009A

摘要： 长伽马射线暴 (GRB) 的高度可变且能量极高的脉冲辐射被认为起源于由大质量恒星坍缩形成的新生致密天体所发射的超相对论喷流内部的动能或磁能的局域快速耗散。GRB 脉冲的光谱最佳拟合为幂律段，表明非热辐射过程占主导地位。人们曾大力搜寻过境后的余辉在 X 射线和软$\gamma$射线波段的光谱线，但从未得到确认。高能瞬时辐射中从未发现线特征。在这里，我们报告了在有史以来最亮的 GRB 221009A 中大约$10$MeV 处发现了一个具有高度显著性的 ($> 6 \sigma$) 窄发射特征。 通过用高斯函数拟合其轮廓，我们发现宽度约为常数$\sigma \sim 1$MeV，并且在能量（$\sim 12$MeV 到$\sim 6$MeV）和光度（$\sim 10^{50}$erg/s 到$\sim 2 \times 10^{49}$erg/s）上都表现出大约80秒的时间演化。 我们将这一特征解释为蓝移的正负电子对湮灭线，这些正负电子对相对冷（$k_\mathrm{B}T\ll m_\mathrm{e}c^2$），可能在伽马射线暴（GRB）最亮脉冲产生的喷流区域内形成。 深入了解能够产生这种特征的条件，可能会揭示迄今为止对GRB喷流性质和能量耗散机制知之甚少的领域。 显示英文摘要

摘要： The highly variable and energetic pulsed emission of a long gamma-ray burst (GRB) is thought to originate from local, rapid dissipation of kinetic or magnetic energy within an ultra-relativistic jet launched by a newborn compact object, formed during the collapse of a massive star. The spectra of GRB pulses are best modelled by power-law segments, indicating the dominance of non-thermal radiation processes. Spectral lines in the X-ray and soft $\gamma$-ray regime for the afterglow have been searched for intensively, but never confirmed. No line features ever been identified in the high energy prompt emission. Here we report the discovery of a highly significant ($> 6 \sigma$) narrow emission feature at around $10$ MeV in the brightest ever GRB 221009A. By modelling its profile with a Gaussian, we find a roughly constant width $\sigma \sim 1$ MeV and temporal evolution both in energy ($\sim 12$ MeV to $\sim 6$ MeV) and luminosity ($\sim 10^{50}$ erg/s to $\sim 2 \times 10^{49}$ erg/s) over 80 seconds. We interpret this feature as a blue-shifted annihilation line of relatively cold ($k_\mathrm{B}T\ll m_\mathrm{e}c^2$) electron-positron pairs, which could have formed within the jet region where the brightest pulses of the GRB were produced. A detailed understanding of the conditions that can give rise to such a feature could shed light on the so far poorly understood GRB jet properties and energy dissipation mechanism.