GW170817 was the first gravitational-wave detection of a binary neutron-star merger. It was accompanied by radiation across the electromagnetic spectrum and localized to the galaxy NGC 4993 at a distance of 40 megaparsecs. It has been proposed that the observed γ-ray, X-ray and radio emission is due to an ultra-relativistic jet being launched during the merger (and successfully breaking out of the surrounding material), directed away from our line of sight (off-axis). The presence of such a jet is predicted from models that posit neutron-star mergers as the drivers of short hard-γ-ray bursts. Here we report that the radio light curve of GW170817 has no direct signature of the afterglow of an off-axis jet. Although we cannot completely rule out the existence of a jet directed away from the line of sight, the observed γ-ray emission could not have originated from such a jet. Instead, the radio data require the existence of a mildly relativistic wide-angle outflow moving towards us. This outflow could be the high-velocity tail of the neutron-rich material that was ejected dynamically during the merger, or a cocoon of material that breaks out when a jet launched during the merger transfers its energy to the dynamical ejecta. Because the cocoon model explains the radio light curve of GW170817, as well as the γ-ray and X-ray emission (and possibly also the ultraviolet and optical emission), it is the model that is most consistent with the observational data. Cocoons may be a ubiquitous phenomenon produced in neutron-star mergers, giving rise to a hitherto unidentified population of radio, ultraviolet, X-ray and γ-ray transients in the local Universe.

中文翻译：

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中子星合并事件 GW170817 中的温和相对论广角流出

GW170817 是对双中子星合并的首次引力波探测。它伴随着整个电磁波谱的辐射，并定位在距离 40 秒差距的星系 NGC 4993 上。有人提出，观测到的 γ 射线、X 射线和无线电发射是由于在合并期间发射的超相对论射流（并成功地突破了周围的物质），远离我们的视线（关闭-轴）。这种喷流的存在是根据假设中子星合并是短时间硬γ射线爆发的驱动因素的模型预测的。在这里，我们报告 GW170817 的射电光曲线没有离轴喷流余辉的直接特征。虽然我们不能完全排除远离视线的喷射流的存在，观察到的 γ 射线发射不可能来自这样的喷流。相反，无线电数据需要存在向我们移动的温和的相对论广角流出。这种流出可能是合并过程中动态喷射出的富含中子物质的高速尾部，或者是合并过程中发射的喷流将其能量转移到动态喷射物时破裂的物质茧。因为茧模型解释了 GW170817 的射电光曲线，以及 γ 射线和 X 射线发射（可能还有紫外线和光发射），它是与观测数据最一致的模型。茧可能是中子星合并中普遍存在的现象，在本地宇宙中产生了迄今为止未知的无线电、紫外线、X 射线和 γ 射线瞬变群。