Millisecond pulsars are neutron stars that attain their very fast rotation during a 108–109-yr-long phase of disk accretion of matter from a low-mass companion star1,2. 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 poles3. 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 radio4 and gamma-ray5 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 identified6,7. 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 magnetosphere8 seems more likely.
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F.A., P.C. and F.M. acknowledge the research project Protocol C26A15YCJ4 funded by the Department of Physics of the University of Rome (La Sapienza), for the financial support. A.P. acknowledges funding from the European Union’s Horizon 2020 Framework Programme for Research and Innovation under the Marie Skłodowska-Curie Individual Fellowship grant agreement 660657-TMSP-H2020-MSCA-IF-2014, and wishes to thank D. de Martino, N. Rea and D. F. Torres for useful discussions. L.B., T.D.S. and A.P. acknowledge fruitful discussion with the international team on ‘The disk–magnetosphere interaction around transitional millisecond pulsars at the International Space Science Institute (ISSI), Bern. L.B., T.D.S., G.L.I. and L.S. acknowledge financial contribution from the agreement ASI-INAF I/037/12/0. Results obtained and presented in this paper are based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. The authors gratefully acknowledge time Discretionary Observing Time granted by the TNG Director, E. Molinari and thank C. Rossi for her scientific support and contribution.
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Nature Astronomy (2017)