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Published June 18, 2024 | Version in press
Journal Article Open

Complex rotational dynamics of the neutron star in Hercules X-1 revealed by X-ray polarization

Creators

  • 1. ROR icon California Institute of Technology

Abstract

In an accreting X-ray pulsar, a neutron star accretes matter from a companion star through an accretion disk. The magnetic field of the rotating neutron star disrupts the inner edge of the disk, funnelling the gas to flow onto the poles on its surface. Hercules X-1 is a prototypical persistent X-ray pulsar about 7 kpc from Earth. Its emission varies on three distinct timescales: the neutron star rotates every 1.2 s, it is eclipsed by its companion each 1.7 d, and the system exhibits a superorbital period of 35 d, which has remained stable since its discovery. Several lines of evidence point to the source of this variation as the precession of the accretion disk or that of the neutron star. Despite the many hints over the past 50 yr, the precession of the neutron star itself has yet not been confirmed or refuted. X-ray polarization measurements (probing the spin geometry of Her X-1) with the Imaging X-ray Polarimetry Explorer suggest that free precession of the neutron star crust sets the 35 d period; this has the important implication that its crust is somewhat asymmetric by a few parts per ten million.

Copyright and License

© The Author(s), under exclusive licence to Springer Nature Limited 2024.

Acknowledgement

IXPE is a joint US and Italian mission. The US contribution is supported by the National Aeronautics and Space Administration (NASA) and led and managed by its Marshall Space Flight Center, with industry partner Ball Aerospace (Contract NNM15AA18C). The Italian contribution is supported by the Italian Space Agency (Contract ASI-ASI-OHBI-2022-13-I.0 and Agreements ASI-INAF-2022-19-HH.0 and ASI-INFN-2017.13-H0) and its Space Science Data Center (Agreements ASI-INAF-2022-14-HH.0 and ASI-INFN 2021-43-HH.0), and by the Italian National Institute for Astrophysics and the Italian National Institute for Nuclear Physics. This research used data products provided by the IXPE Team (Marshall Space Flight Center, Space Science Data Center, Italian National Institute for Astrophysics and Italian National Institute for Nuclear Physics) and distributed with additional software tools by the High-Energy Astrophysics Science Archive Research Center at NASA’s Goddard Space Flight Center. J.H. acknowledges support from the Natural Sciences and Engineering Research Council of Canada through a discovery grant, the Canadian Space Agency through the co-investigator grant program, and computational resources and services provided by Compute Canada, Advanced Research Computing at the University of British Columbia, and the SciServer science platform (www.sciserver.org). D.G.-C. acknowledges support from a fellowship grant from the French National Centre for Space Studies. J.P. and S.S.T. were supported by the Academy of Finland (Grant Nos. 333112 and 349144) and the Väisälä Foundation. V.D. and V.F.S. thank the German Academic Exchange Service (Travel Grant No. 57525212). We used Astropy (http://www.astropy.org), a community-developed core Python package and an ecosystem of tools and resources for astronomy.

Contributions

J.H. analysed the data and wrote the draft of the paper. J.P. led the work of the IXPE Topical Working Group on Accreting Neutron Stars and contributed to the interpretation and the text. V.D., D.G.-C., I.C., A. Mushtukov., S.S.T., D.M. and V.F.S. contributed to the interpretation of the results and writing of the text. A. Mushtukov created Fig. 3. M.B. and G.G.P. acted as internal referees of the paper and contributed to its interpretation. Other members of the IXPE collaboration contributed to the design of the mission and its science case and the planning of the observations. All authors provided input and comments on the paper.

Data Availability

The data used for this analysis are available through High-Energy Astrophysics Science Archive Research Center under IXPE Observation IDs 01001899, 02003801 and 02004001.

Extended Data Fig. 1 Posteriors for the RVM for the First Main-On (2022 January).

Extended Data Fig. 2 Posteriors for the RVM for the First Main-On (early).

Extended Data Fig. 3 Posteriors for the RVM for the First Main-On (late).

Extended Data Fig. 4 Posteriors for the RVM for the Short-On (2023 January).

Extended Data Fig. 5 Posteriors for the RVM for the Second Main- On (2023 February).

Supplementary Figs. 1–3

Code Availability

The software used for this analysis is available at https://github.com/UBC-Astrophysics/IXPE-Analysis.

Conflict of Interest

The authors declare that they have no competing financial interests.

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Additional details

Identifiers

URL
https://rdcu.be/dLrrL

Funding

National Aeronautics and Space Administration
NNM15AA18C
Agenzia Spaziale Italiana
ASI-ASI-OHBI-2022-13-I.0
Agenzia Spaziale Italiana
ASI-INAF-2022-19-HH.0
Agenzia Spaziale Italiana
ASI-INFN-2017.13-H0
Agenzia Spaziale Italiana
ASI-INAF-2022-14-HH.0
Agenzia Spaziale Italiana
ASI-INFN 2021-43-HH.0
National Institute for Astrophysics
Istituto Nazionale di Fisica Nucleare
Natural Sciences and Engineering Research Council
Canadian Space Agency
Compute Canada
Centre National d'Études Spatiales
Research Council of Finland
333112
Research Council of Finland
349144
Vaisala (Finland)
German Academic Exchange Service
57525212

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