The discovery of a radioactively powered kilonova associated with the binary neutron-star merger GW170817 remains the only confirmed electromagnetic counterpart to a gravitational-wave event 1 , 2 . Observations of the late-time electromagnetic emission, however, do not agree with the expectations from standard neutron-star merger models. Although the large measured ejecta mass 3 , 4 could be explained by a progenitor system that is asymmetric in terms of the stellar component masses (that is, with a mass ratio q of 0.7 to 0.8) 5 , the known Galactic population of merging double neutron-star systems (that is, those that will coalesce within billions of years or less) has until now consisted only of nearly equal-mass ( q > 0.9) binaries 6 . The pulsar PSR J1913+1102 is a double system in a five-hour, low-eccentricity (0.09) orbit, with an orbital separation of 1.8 solar radii 7 , and the two neutron stars are predicted to coalesce in $${470}_{-11}^{+12}$$ 470 − 11 + 12 million years owing to gravitational-wave emission. Here we report that the masses of the pulsar and the companion neutron star, as measured by a dedicated pulsar timing campaign, are 1.62 ± 0.03 and 1.27 ± 0.03 solar masses, respectively. With a measured mass ratio of q = 0.78 ± 0.03, this is the most asymmetric merging system reported so far. On the basis of this detection, our population synthesis analysis implies that such asymmetric binaries represent between 2 and 30 per cent (90 per cent confidence) of the total population of merging binaries. The coalescence of a member of this population offers a possible explanation for the anomalous properties of GW170817, including the observed kilonova emission from that event. Pulsar timing measurements show a mass ratio of about 0.8 for the double neutron-star system PSR J1913+1102, and population synthesis models indicate that such asymmetric systems represent 2–30% of merging binaries.

中文翻译：

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明亮的双中子星合并的不对称质量比

与双中子星合并 GW170817 相关的放射性驱动千新星的发现仍然是唯一确认的引力波事件的电磁对应物 1 , 2 。然而，对后期电磁辐射的观察与标准中子星合并模型的预期不一致。尽管测得的大喷射物质量 3 , 4 可以用恒星组成质量不对称的前身系统（即质量比 q 为 0.7 到 0.8） 5 来解释，但已知的合并双中子的银河族群-星系统（即那些将在数十亿年或更短的时间内合并的系统）迄今为止仅由质量几乎相等（q > 0.9）的双星 6 组成。脉冲星 PSR J1913+1102 是一个在 5 小时低偏心率 (0.09) 轨道上运行的双系统，轨道间隔为 1.8 个太阳半径 7 ，由于引力波发射，两颗中子星预计将在 $${470}_{-11}^{+12}$$470 − 11 + 1200 万年中合并. 在这里，我们报告了脉冲星和伴生中子星的质量，由专门的脉冲星计时运动测量，分别为 1.62 ± 0.03 和 1.27 ± 0.03 太阳质量。测量的质量比为 q = 0.78 ± 0.03，这是迄今为止报道的最不对称的合并系统。在此检测的基础上，我们的总体综合分析表明，这种不对称双星代表了合并双星总人口的 2% 到 30%（置信度为 90%）。该种群成员的融合为 GW170817 的异常特性提供了可能的解释，包括从该事件中观测到的千新星发射。脉冲星计时测量显示双中子星系统 PSR J1913+1102 的质量比约为 0.8，群体合成模型表明这种不对称系统占合并双星的 2-30%。