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Transient Be/X-ray binary systems, wherein a compact object accretes from aBe-companion star, can show giant and periastron outbursts. During the decay oftheir giant outbursts, some Be/X-ray binaries also show X-ray re-brightenings,the origin of which is not understood. Recently, we presented the discovery ofa jet from a neutron star Be/X-ray binary, observed during the giant outburstof Swift J0243.6+6124. Here, we present continued radio monitoring of its2017/2018 giant outburst decay and a re-brightening of this source. During theformer, we observe a radio flare with a steep radio spectrum, possibly causedby interactions between discrete ejecta colliding with the pre-existing jet orthe surrounding medium. During the X-ray re-brightening, we observe the radiojet turning on and off within days. Surprisingly, this re-establishing jet isas bright in radio as at the peak of the super-Eddington giant outburst,despite more than two orders of magnitude lower X-ray luminosity. In addition,the jet is only observed when the X-ray luminosity exceeds approximately$2\times 10^{36} (D/5\rm kpc)^2$ erg/s. We discuss how such an X-ray thresholdfor jet launching might be related to the presence of a magnetic centrifugalbarrier at lower mass accretion rates. We also discuss the implications of ourresults for the launch of jets from strongly magnetized neutron stars, andexplore future avenues to exploit the new possibility of coordinatedX-ray/radio studies of neutron star Be/X-ray binaries.

A re-establishing jet during an X-ray re-brightening of the Be/X-ray binary Swift J0243.6+6124