  Millisecond spinning, low magnetic field neutron stars are believed to attain
their fast rotation in a 0.1-1 Gyr-long phase during which they accrete matter
endowed with angular momentum from a low-mass companion star. Despite extensive
searches, coherent periodicities originating from accreting neutron star
magnetospheres have been detected only at X-ray energies and in ~10% of the
presently known systems. Here we report the detection of optical and
ultraviolet coherent pulsations at the X-ray period of the transient low mass
X-ray binary system SAX J1808.4-3658, during an accretion outburst that
occurred in August 2019. At the time of the observations, the pulsar was
surrounded by an accretion disc, displayed X-ray pulsations and its luminosity
was consistent with magnetically funneled accretion onto the neutron star.
Current accretion models fail to account for the luminosity of both optical and
ultraviolet pulsations; these are instead more likely driven by
synchro-curvature radiation in the pulsar magnetosphere or just outside of it.
This interpretation would imply that particle acceleration can take place even
when mass accretion is going on, and opens up new perspectives in the study of
coherent optical/UV pulsations from fast spinning accreting neutron stars in
low-mass X-ray binary systems.
