Optical pulsations from a transitional millisecond pulsar

Millisecond pulsars are neutron stars that attain their very fast rotation during a 10 8 –10 9 -yr-long phase of disk accretion of matter from a low-mass companion star 1 , 2 . They can be detected as accretion-powered millisecond X-ray pulsars if towards the end of this phase their magnetic field is strong enough to channel the in-flowing matter towards their magnetic poles 3 . When mass transfer is reduced or ceases altogether, pulsed emission generated by magnetospheric particle acceleration and powered by the star rotation is observed, preferentially in the radio 4 and gamma-ray 5 bands. A few transitional millisecond pulsars that swing between an accretion-powered X-ray pulsar regime and a rotationally powered radio pulsar regime in response to variations of the mass in-flow rate have been recently identified 6 , 7 . Here, we report the detection of optical pulsations from a transitional millisecond pulsar. The pulsations were observed when the pulsar was surrounded by an accretion disk, and originated inside the magnetosphere or within a few hundreds of kilometres from it. Energy arguments rule out reprocessing of accretion-powered X-ray emission and argue against a process related to accretion onto the pulsar polar caps; synchrotron emission of electrons in a rotation-powered pulsar magnetosphere 8 seems more likely. Optical pulsations from a millisecond pulsar that had transitioned from a rotationally powered regime to an accretion disk state have been detected. The optical emission is likely to be due to electron synchrotron emission in a rotation-powered magnetosphere.