Optical pulsations from a transitional millisecond pulsar

TL;DR

This paper reports the first detection of optical pulsations from a transitional millisecond pulsar, revealing insights into neutron star magnetospheres during accretion phases.

Contribution

It presents the first observation of optical pulsations from a millisecond pulsar in a transitional state, expanding understanding of pulsar emission mechanisms.

Findings

Optical pulsations detected from a transitional millisecond pulsar.

Pulsations originate near the magnetosphere, not from accretion hotspots.

Synchrotron emission in the magnetosphere is the likely source.

Abstract

Weakly magnetic, millisecond spinning neutron stars attain their very fast rotation through a 1E8-1E9 yr long phase during which they undergo disk-accretion of matter from a low mass companion star. They can be detected as accretion-powered millisecond X-ray pulsars if towards the end of this phase their magnetic field is still strong enough to channel the accreting matter towards the magnetic poles. When mass transfer is much reduced or ceases altogether, pulsed emission generated by particle acceleration in the magnetosphere and powered by the rotation of the neutron star is observed, preferentially in the radio and gamma-ray 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. Here we report the detection of optical pulsations from a transitional pulsar, the first ever from a millisecond spinning neutron star. 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 seems more likely.