  Relativistic jets are observed from accreting and cataclysmic transients
throughout the Universe, and have a profound affect on their surroundings.
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 or in the star
itself, but to-date no such measurements exist. These objects can display
bright explosions on their surface due to unstable thermonuclear burning of
recently accreted material, called type-I X-ray bursts, during which the mass
accretion rate increases. 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 $v=0.38^{+0.11}_{-0.08}$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.
