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# Optical and ultraviolet pulsed emission from an accreting millisecond pulsar

## 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 star1. Despite extensive searches, coherent periodicities originating from accreting neutron star magnetospheres have been detected only at X-ray energies2 and in ~10% of the currently known systems3. 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 20194. At the time of the observations, the pulsar was surrounded by an accretion disk, displayed X-ray pulsations and its luminosity was consistent with magnetically funnelled 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 to be driven by synchro-curvature radiation5,6 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/ultraviolet pulsations from fast-spinning accreting neutron stars in low-mass X-ray binary systems.

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## Data availability

Source data are provided with this paper. The barycentered SiFAP2 data that support the findings of this study are available in figshare at https://doi.org/10.6084/m9.figshare.12707444.

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## Acknowledgements

A.M.Z. thanks the HST contact scientist, D. Welty (STScI), for constant support in the observation planning and T. Royle (STScI) for checking the scheduling processes. A.M.Z. thanks A. Riley (STIS Team) for the support in the scientific data analysis. A.M.Z. acknowledges the support of the PHAROS COST Action (CA16214) and A. Ridolfi for his help in data analysis. A.M.Z. would also like to thank G. Benevento for comments on draft. F.C.Z. is supported by a Juan de la Cierva fellowship. S.C. and P.D.A. acknowledge support from ASI grant I/004/11/3. D.d.M., A.P. and L.S. acknowledge financial support from the Italian Space Agency (ASI) and National Institute for Astrophysics (INAF) under agreements ASI-INAF I/037/12/0. L.B., D.d.M., T.D.S., A.P. and L.S. acknowledge financial contributions from ASI-INAF agreement no. 2017-14-H.0, INAF main-stream (principal investigator: T. Belloni; principal investigator: A. De Rosa). D.F.T. acknowledges support from grants PGC2018-095512-B-I00, SGR2017-1383 and AYA2017-92402-EXP. L.B and T.D.S. thank A. Marino for useful discussions and acknowledge financial contributions from the HERMES project financed by the Italian Space Agency (ASI) agreement no. 2016/13 U.O. T.D.S. and L.S. acknowledge the iPeska research grant (principal investigator: A. Possenti) funded under the INAF national call Prin-SKA/CTA approved with the Presidential Decree 70/2016. A.P., F.C.Z., and D.T. acknowledge the International Space Science Institute (ISSI-Beijing), which funded and hosted the international team ‘Understanding and Unifying the Gamma-rays Emitting Scenarios in High Mass and Low Mass X-ray Binaries’. Results obtained with SiFAP2 and presented in this paper are based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated by the Fundación Galileo Galilei (FGG) of the Istituto Nazionale di Astrofisica (INAF) at the Observatorio del Roque de los Muchachos (La Palma, Canary Islands, Spain). Part of this paper is based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. This work also made use of data and software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC).

## Author information

Authors

### Contributions

F.A., A.M.Z., A.P. and F.C.Z. analysed optical, UV and X-ray data. F.A., A.M.Z., A.P., F.C.Z. and L.S. wrote the paper. A.M.Z., A.P., S.C., P.D.A., F.C.Z., P.C., L.S., T.D.S., L.B., D.d.M., D.F.T., G.L.I. and A.S. interpreted the results. F.A., F.M., P. Cretaro, A.G., F. Leone. and E.P. conceived SiFAP2. A.G., A.P. and F.A. performed the optical observation. A.G., M. Cecconi, M.D.G.G., A.L.R.R., H.P.V., M.H.D. and J.J.S.J. developed the SiFAP2 mechanical interface and its relative control software. M. Cadelano and R.P.M. contributed to the HST data analysis. M.C.B., D.M.R., D.M.B. and F. Lewis. contributed to the optical part of the SED. All authors read, commented on and approved the submission of this article.

### Corresponding authors

Correspondence to F. Ambrosino or A. Miraval Zanon.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review informationNature Astronomy thanks Lucien Kuiper and Jumpei Takata for their contribution to the peer review of this work.

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## Supplementary information

### Supplementary Information

Supplementary Figs. 1–7 and Tables 1–2.

## Source data

### Source Data Fig. 1

NICER X-ray light curve of SAX J1808.4-3658 data.

### Source Data Fig. 2

Fast Fourier transform of both optical and UV datasets. Optical and UV pulse profiles’ data (inset of Fig. 2).

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Ambrosino, F., Miraval Zanon, A., Papitto, A. et al. Optical and ultraviolet pulsed emission from an accreting millisecond pulsar. Nat Astron 5, 552–559 (2021). https://doi.org/10.1038/s41550-021-01308-0

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• DOI: https://doi.org/10.1038/s41550-021-01308-0