Optical and ultraviolet pulsed emission from an accreting millisecond pulsar

TL;DR

This paper reports the first detection of optical and ultraviolet pulsations from an accreting millisecond pulsar during an outburst, suggesting new mechanisms for particle acceleration in such systems.

Contribution

It presents the first observation of optical/UV pulsations at the X-ray period in an accreting millisecond pulsar, challenging existing accretion models.

Findings

Optical and UV pulsations detected during an accretion outburst.

Pulsations likely driven by synchro-curvature radiation outside the magnetosphere.

Current models cannot fully explain the luminosity of observed pulsations.

Abstract

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.