Thermonuclear explosions on neutron stars reveal the speed of their jets.

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 flow 3 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 bursts 6 , during which the mass-accretion rate increases 7-9 . 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.