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Gravitationally redshifted absorption lines in the X-ray burst spectra of a neutron star

Nature volume 420, pages 51–54 (2002)Cite this article

Abstract

The fundamental properties of neutron stars provide a direct test of the equation of state of cold nuclear matter, a relationship between pressure and density that is determined by the physics of the strong interactions between the particles that constitute the star. The most straightforward method of determining these properties is by measuring the gravitational redshift of spectral lines produced in the neutron star photosphere1. The equation of state implies a mass–radius relation, while a measurement of the gravitational redshift at the surface of a neutron star provides a direct constraint on the mass-to-radius ratio. Here we report the discovery of significant absorption lines in the spectra of 28 bursts of the low-mass X-ray binary EXO0748-676. We identify the most significant features with the Fe xxvi and xxv n = 2–3 and O viii n = 1–2 transitions, all with a redshift of z = 0.35, identical within small uncertainties for the respective transitions. For an astrophysically plausible range of masses (M ≈ 1.3–2.0 solar masses; refs 2–5), this value is completely consistent with models of neutron stars composed of normal nuclear matter, while it excludes some models6,7 in which the neutron stars are made of more exotic matter.

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Figure 1: The XMM-Newton RGS spectra of EXO0748-676 for 28 type I X-ray bursts.

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References

  1. van Paradijs, J. Possible observational constraints on the mass-radius relation of neutron stars. Astrophys. J. 234, 609–611 (1979)

    Article  ADS  Google Scholar 

  2. Thorsett, S. E. & Chakrabarty, D. Neutron star mass measurements. I. Radio pulsars. Astrophys. J. 512, 288–299 (1999)

    Article  ADS  Google Scholar 

  3. Barziv, O., Kaper, L., Van Kerkwijk, M. H., Telting, J. H. & Van Paradijs, J. The mass of the neutron star in Vela X-1. Astron. Astrophys. 377, 925–944 (2001)

    Article  ADS  CAS  Google Scholar 

  4. Orosz, J. A. & Kuulkers, E. The optical light curves of Cygnus X-2 (V1341 Cyg) and the mass of its neutron star. Mon. Not. R. Astron. Soc. 305, 132–142 (1999)

    Article  ADS  Google Scholar 

  5. Zhang, W., Smale, A. P., Strohmayer, T. E. & Swank, J. H. Correlation between energy spectral states and fast time variability and further evidence for the marginally stable orbit in 4U 1820 - 30. Astrophys. J. 500, L171–L174 (1998)

    Article  ADS  Google Scholar 

  6. Lattimer, J. M. & Prakash, M. Neutron star structure and the equation of state. Astrophys. J. 550, 426–442 (2001)

    Article  ADS  Google Scholar 

  7. Dey, M., Bombaci, I., Dey, J., Ray, S. & Samanta, B. C. Strange stars with realistic quark vector interaction and phenomenological density-dependent scalar potential. Phys. Lett. B 438, 123–128 (1998) ; erratum 467, 303–305 (1999)

    Article  ADS  CAS  Google Scholar 

  8. Jansen, F. et al. The XMM-Newton observatory. I. The spacecraft and operations. Astron. Astrophys. 365, L1–L6 (2001)

    Article  ADS  Google Scholar 

  9. Parmar, A. N., White, N. E., Giommi, P. & Gottwald, M. The discovery of 3.8 hour periodic intensity dips and eclipses from the transient low-mass X-ray binary EXO 0748 - 676. Astrophys. J. 308, 199–212 (1986)

    Article  ADS  CAS  Google Scholar 

  10. den Herder, J. W. et al. The Reflection Grating Spectrometer on board XMM-Newton. Astron. Astrophys. 365, L7–L17 (2001)

    Article  ADS  CAS  Google Scholar 

  11. Turner, M. J. L. et al. The European Photon Imaging Camera on XMM-Newton: The MOS cameras. Astron. Astrophys. 365, L27–L35 (2001)

    Article  ADS  Google Scholar 

  12. Strüder, L. et al. The European Photon Imaging Camera on XMM-Newton: The pn-CCD camera. Astron. Astrophys. 365, L18–L26 (2001)

    Article  ADS  Google Scholar 

  13. Cottam, J., Kahn, S. M., Brinkman, A. C., den Herder, J. W. & Erd, C. High-resolution spectroscopy of the low-mass X-ray binary EXO 0748 - 67. Astron. Astrophys. 365, L277–L281 (2001)

    Article  ADS  CAS  Google Scholar 

  14. Lewin, W. H. G., van Paradijs, J. & Taam, R. E. X-Ray bursts. Space Sci. Rev. 62, 223 (1993)

    Article  ADS  Google Scholar 

  15. Sako, M. et al. Complex resonance absorption structure in the X-ray spectrum of IRAS 13349 + 2438. Astron. Astrophys. 365, L168–L173 (2001)

    Article  ADS  CAS  Google Scholar 

  16. Paerels, F. et al. Interstellar X-ray absorption spectroscopy of oxygen, neon, and iron with the CHANDRA LETGS spectrum of X0614 + 091. Astrophys. J. 546, 338–344 (2001)

    Article  ADS  CAS  Google Scholar 

  17. Tarter, C. B., Tucker, W. H. & Salpeter, E. E. The interaction of X-ray sources with optically thin environments. Astrophys. J. 156, 943–952 (1969)

    Article  ADS  CAS  Google Scholar 

  18. Kallman, T. R. & McCray, R. X-ray nebular models. Astrophys. J. Suppl. Ser. 50, 263–317 (1982)

    Article  ADS  CAS  Google Scholar 

  19. Wijnands, R. & van der Klis, M. A millisecond pulsar in an X-ray binary system. Nature 394, 344–346 (1998)

    Article  ADS  CAS  Google Scholar 

  20. Galloway, D. K., Chakrabarty, D., Morgan, E. H. & Remillard, R. A. Discovery of a high-latitude accreting millisecond pulsar in an ultracompact binary. Astrophys. J. 576, L137–L140 (2002)

    Article  ADS  Google Scholar 

  21. Psaltis, D. & Chakrabarty, D. The disk-magnetosphere interaction in the accretion-powered millisecond pulsar SAX J1808.4-3658. Astrophys. J. 521, 332–340 (1999)

    Article  ADS  Google Scholar 

  22. Sanwal, D., Pavlov, G. G., Zavlin, V. E. & Teter, M. A. Discovery of absorption features in the X-ray spectrum of an isolated neutron star. Astrophys. J. 574, L61–L64 (2002)

    Article  ADS  CAS  Google Scholar 

  23. Waki, I. et al. Discovery of absorption lines in X-ray burst spectra from X1636-536. Publ. Astron. Soc. Jpn 36, 819–830 (1984)

    ADS  CAS  Google Scholar 

  24. Sztajno, M. et al. Unusual X-ray burst profiles from 4U/MXB 1636-53. Astrophys. J. 299, 487–495 (1985)

    Article  ADS  CAS  Google Scholar 

  25. Nakamura, N., Inoue, H. & Tanaka, Y. Detection of absorption lines in the spectra of X-ray bursts from X1068-52. Publ. Astron. Soc. Jpn 40, 209–217 (1988)

    ADS  CAS  Google Scholar 

  26. Magnier, E. et al. A 4.1 keV spectral feature in a type 1 X-ray burst from EXO 1747-214. Mon. Not. R. Astron. Soc. 237, 729–738 (1989)

    Article  ADS  CAS  Google Scholar 

  27. Bethe, H. A. & Salpeter, E. E. Quantum Mechanics of One and Two Electron Atoms (Plenum, New York, 1977)

    Book  Google Scholar 

  28. Kudritzki, R. P. & Hummer, D. G. Quantitative spectroscopy of hot stars. Annu. Rev. Astron. Astrophys. 28, 303–345 (1990)

    Article  ADS  CAS  Google Scholar 

Download references

Acknowledgements

This work is based on observations obtained with the XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and the USA (NASA). We thank E. Behar for supplying us with results from his atomic-structure calculations of the He-like Fe ion, and M. Sako for the use of his absorption spectral code.

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Authors and Affiliations

  1. NASA Goddard Space Flight Center, Laboratory for High Energy Astrophysics, Greenbelt, Maryland, 20771, USA

    J. Cottam

  2. Columbia Astrophysics Laboratory and Department of Astronomy, Columbia University, 538 West 120th Street, New York, New York, 10027, USA

    F. Paerels

  3. SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands

    M. Mendez

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  1. J. Cottam
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  3. M. Mendez
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Correspondence to J. Cottam.

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Cottam, J., Paerels, F. & Mendez, M. Gravitationally redshifted absorption lines in the X-ray burst spectra of a neutron star. Nature 420, 51–54 (2002). https://doi.org/10.1038/nature01159

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