Relativistic jets are observed from accreting and cataclysmic transients throughout the Universe, and have a profound impact on their surroundings^1,2. Despite their importance, their launch mechanism is not known. For accreting neutron stars, the speed of their compact jets can reveal whether the jets are powered by magnetic fields anchored in the accretion flow³ or in the star itself^4,5, but so far no such measurements exist. These objects can show bright explosions on their surface due to unstable thermonuclear burning of recently accreted material, called type-I X-ray bursts⁶, during which the mass-accretion rate increases^7,8,9. Here, we report on bright flares in the jet emission for a few minutes after each X-ray burst, attributed to the increased accretion rate. With these flares, we measure the speed of a neutron star compact jet to be \(v={0.38}_{-0.08}^{+0.11}c\), much slower than those from black holes at similar luminosities. This discovery provides a powerful new tool in which we can determine the role that individual system properties have on the jet speed, revealing the dominant jet launching mechanism.

相对论性喷流是从整个宇宙的吸积和灾难性瞬变中观察到的，并对周围环境产生深远的影响^1,2。尽管它们很重要，但它们的发射机制尚不清楚。对于吸积中子星，其紧凑喷流的速度可以揭示喷流是否由锚定在吸积流³或恒星本身^4,5中的磁场提供动力，但到目前为止还没有这样的测量。由于最近吸积物质的不稳定热核燃烧，这些物体可以在其表面显示明亮的爆炸，称为 I 型 X 射线爆发⁶，在此期间质量吸积率增加^7,8,9。在这里，我们报告了每次 X 射线爆发后几分钟喷射流发射的明亮耀斑，这归因于吸积率的增加。通过这些耀斑，我们测量出中子星致密喷流的速度为\(v={0.38}_{-0.08}^{+0.11}c\)，比类似光度下黑洞的速度慢得多。这一发现提供了一个强大的新工具，我们可以通过该工具确定各个系统特性对喷射速度的作用，从而揭示主要的喷射发射机制。