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Optical pulsations from a transitional millisecond pulsar

Nature Astronomy volume 1, pages 854–858 (2017)Cite this article

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Abstract

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.

PSR J1023+0038 is a 1.69-ms-spinning neutron star in a 4.75-h orbit around a 0.2 M ⊙ main-sequence-like companion, located at a distance9 of 1.37 kpc. It was discovered in 2008 as a rotationally powered radio pulsar6 releasing a spin-down power9 of 4.3 × 1034 erg s−1, corresponding to a surface magnetic field of ≃108 G. In June 2013, radio pulsations disappeared and an accretion disk developed, accompanied by a factor of ~10 increase both in the average10,11,12 and pulsed X-ray flux13,14 as compared with the radio pulsar regime. This increase was ascribed to channelled mass accretion onto the pulsar polar caps, even though the X-ray luminosity15 (≃7 × 1033 erg s−1, 0.3–79 keV) was considerably lower than both the luminosity of accreting millisecond pulsars in outburst and the luminosity corresponding to the mass accretion rate required to overcome the centrifugal barrier of the pulsar rotating magnetosphere and its propelling effect (assuming matter reaches the neutron star). Its gamma-ray10 and flat-spectrum continuum radio emission, consistent with a jet-like outflow16, add to the complex phenomenology of PSR J1023+0038 in the accretion-disk state, possibly being manifestations of the interaction between the disk, the pulsar magnetosphere and its wind10,15,17,18. Archival observations6 indicate that PSR J1023+0038 was in a similar state in 2001. The optical counterpart of PSR J1023+0038, AY Sex, in the disk state is a \(g\simeq 16.7\) mag blue object that emits an average luminosity18,19 of \({L}_{\text{opt}}\simeq {10}^{33}\) erg s−1 in the 320–900-nm band. Most of the optical/UV emission originates from the outer regions of the disk, and from the companion star’s face illuminated by the pulsar’s high-energy radiation, which drives a \(\Delta g\simeq 0.4\) mag modulation18 of the optical flux at the orbital period of the system.

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Fig. 1: Coherent optical pulsations of PSR J1023+0038.
Fig. 2: Variability of the coherent optical pulsed amplitude of PSR J1023+0038.
Fig. 3: Optical efficiency of rotation-powered pulsars, \({\eta }_{{\rm{o}}{\rm{p}}{\rm{t}}}={L}_{opt}\,/\,{\dot{E}}_{sd}\) as a function of the pulsar spin down power, \({\dot{{E}}}_{sd}\).

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Acknowledgements

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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Author notes

  1. F. Ambrosino and A. Papitto contributed equally to this work.

Authors and Affiliations

  1. Dipartimento di Fisica, Università di Roma “La Sapienza”, Piazzale Aldo Moro, 5, I-00185, Roma, Italy

    F. Ambrosino & F. Meddi

  2. INAF - Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, I-00133, Roma, Italy

    F. Ambrosino

  3. INAF - Osservatorio Astronomico di Roma, Via Frascati, 33, I-00078, Monte Porzio Catone (RM), Italy

    A. Papitto, L. Stella & G. L. Israel

  4. INFN - Sezione di Roma 1, Piazzale Aldo Moro, 5, I-00185, Roma, Italy

    P. Cretaro

  5. Dipartimento di Fisica, Università di Cagliari, SP Monserrato-Sestu, Km 0.7, I-09042, Monserrato, Italy

    L. Burderi

  6. Dipartimento di Fisica e Chimica, Università di Palermo, via Archirafi 36, I-90124, Palermo, Italy

    T. Di Salvo

  7. Fundación Galileo Galilei - INAF, Rambla José Ana Fernández Pérez, 7, E-38712, Breña Baja, TF, Spain

    A. Ghedina, L. Di Fabrizio & L. Riverol

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Contributions

F.A. and A.P. jointly coordinated and contributed equally to this study. F.M., F.A. and P.C. conceived and designed the optical photometer. F.A., A.G., L.D.F. and L.R. performed the optical observations. A.P. and F.A. analysed the data. A.P. and L.S. wrote the paper. A.P., L.S., T.D.S. and L.B. interpreted the results. All authors read, commented on and approved submission of this article.

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Correspondence to A. Papitto.

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Ambrosino, F., Papitto, A., Stella, L. et al. Optical pulsations from a transitional millisecond pulsar. Nat Astron 1, 854–858 (2017). https://doi.org/10.1038/s41550-017-0266-2

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