Letter | Published:

A magnetar-powered X-ray transient as the aftermath of a binary neutron-star merger

Naturevolume 568pages198201 (2019) | Download Citation


Mergers of neutron stars are known to be associated with short γ-ray bursts1,2,3,4. If the neutron-star equation of state is sufficiently stiff (that is, the pressure increases sharply as the density increases), at least some such mergers will leave behind a supramassive or even a stable neutron star that spins rapidly with a strong magnetic field5,6,7,8 (that is, a magnetar). Such a magnetar signature may have been observed in the form of the X-ray plateau that follows up to half of observed short γ-ray bursts9,10. However, it has been expected that some X-ray transients powered by binary neutron-star mergers may not be associated with a short γ-ray burst11,12. A fast X-ray transient (CDF-S XT1) was recently found to be associated with a faint host galaxy, the redshift of which is unknown13. Its X-ray and host-galaxy properties allow several possible explanations including a short γ-ray burst seen off-axis, a low-luminosity γ-ray burst at high redshift, or a tidal disruption event involving an intermediate-mass black hole and a white dwarf13. Here we report a second X-ray transient, CDF-S XT2, that is associated with a galaxy at redshift z = 0.738 (ref. 14). The measured light curve is fully consistent with the X-ray transient being powered by a millisecond magnetar. More intriguingly, CDF-S XT2 lies in the outskirts of its star-forming host galaxy with a moderate offset from the galaxy centre, as short γ-ray bursts often do15,16. The estimated event-rate density of similar X-ray transients, when corrected to the local value, is consistent with the event-rate density of binary neutron-star mergers that is robustly inferred from the detection of the gravitational-wave event GW170817.

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We thank A. Lien, K. Hurley and D. Svinkin for examining the relevant Swift/BAT, IPN3, and Wind/KNOUS data, respectively, for us. Y.Q.X., X.C.Z., J.Y.L. and M.Y.S. acknowledge support from the 973 Program (2015CB857004), the National Natural Science Foundation of China (grant numbers NSFC-11473026, NSFC-11890693 and NSFC-11421303), the CAS Frontier Science Key Research Program (QYZDJ-SSW-SLH006), the China Scholarship Council (CSC)-Leiden University Joint Scholarship Program, the China Postdoctoral Science Foundation (2016M600485), and the K. C. Wong Education Foundation. Y.L. acknowledges support by the KIAA-CAS Fellowship, which is jointly supported by Peking University and the Chinese Academy of Sciences. W.N.B. and G.Y. acknowledge support from Chandra X-ray Center grant AR8-19016X, NASA grant NNX17AF07G, NASA ADP grant 80NSSC18K0878 and the V. M. Willaman Endowment. B.L. acknowledges support from the National Key R&D Program of China (2016YFA0400702) and NSFC-11673010. B.-B.Z. acknowledges support from the National Thousand Young Talents programme of China and the National Key Research and Development Program of China (2018YFA0404204) and NSFC-11833003. F.E.B. acknowledges support from CONICyT-Chile (Basal AFB-170002, FONDECyT Regular 1141218, FONDO ALMA 31160033) and the Ministry of Economy, Development, and Tourism’s Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics (MAS). F.V. acknowledges financial support from CONICyT and CASSACA through the fourth call for tenders of the CAS-CONICyT fund.

Author information


  1. CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, China

    • Y. Q. Xue
    • , X. C. Zheng
    • , X. Kong
    • , J. Y. Li
    • , M. Y. Sun
    •  & J.-X. Wang
  2. School of Astronomy and Space Science, University of Science and Technology of China, Hefei, China

    • Y. Q. Xue
    • , X. C. Zheng
    • , X.-F. Wu
    • , X. Kong
    • , J. Y. Li
    • , M. Y. Sun
    •  & J.-X. Wang
  3. Leiden Observatory, Leiden University, Leiden, The Netherlands

    • X. C. Zheng
  4. Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, China

    • Y. Li
  5. Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA, USA

    • W. N. Brandt
    •  & G. Yang
  6. Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA, USA

    • W. N. Brandt
    •  & G. Yang
  7. Department of Physics, The Pennsylvania State University, University Park, PA, USA

    • W. N. Brandt
  8. Department of Physics and Astronomy, University of Nevada, Las Vegas, NV, USA

    • B. Zhang
  9. National Astronomical Observatories, Chinese Academy of Sciences, Beijing, China

    • B. Zhang
    •  & H. Sun
  10. Department of Astronomy, School of Physics, Peking University, Beijing, China

    • B. Zhang
  11. School of Astronomy and Space Science, Nanjing University, Nanjing, China

    • B. Luo
    •  & B.-B. Zhang
  12. Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Nanjing, China

    • B. Luo
    •  & B.-B. Zhang
  13. Collaborative Innovation Center of Modern Astronomy and Space Exploration, Nanjing, China

    • B. Luo
    •  & B.-B. Zhang
  14. Instituto de Astrofísica and Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile, Santiago, Chile

    • F. E. Bauer
    •  & F. Vito
  15. Millennium Institute of Astrophysics (MAS), Santiago, Chile

    • F. E. Bauer
  16. Space Science Institute, Boulder, CO, USA

    • F. E. Bauer
  17. Department of Physics, University of Arkansas, Fayetteville, AR, USA

    • B. D. Lehmer
  18. Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China

    • X.-F. Wu
  19. CAS South America Center for Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing, China

    • F. Vito


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Y.Q.X. and X.C.Z. discovered CDF-S XT2, and led the Chandra data reduction and analysis, estimation of rise time and event rate density, and compilation and analysis of multiwavelength properties, with inputs from W.N.B., B.L., F.E.B., B.D.L., G.Y., X.K., J.Y.L., M.Y.S., J.-X.W. and F.V. B.Z. and X.-F.W. proposed the theoretical model to explain the data. Y.Q.X., X.C.Z. and B.Z. wrote the manuscript. Y.L. analysed the HST data, measured the offset with inputs from X.C.Z. and Y.Q.X., computed the Flight parameter and the probability O(II:I)host, and compared X-ray and γ-ray properties with those of SGRBs. B.-B.Z. analysed the Fermi data and computed relevant flux and luminosity upper limits; B.-B.Z. and X.C.Z. examined the INTEGRAL and Swift data. H.S. calculated the local event rate density with inputs from X.C.Z. and Y.Q.X. All authors contributed to the manuscript.

Competing interests

The authors declare no competing interests.

Corresponding authors

Correspondence to Y. Q. Xue or X. C. Zheng or B. Zhang.

Extended data figures and tables

  1. Extended Data Fig. 1 Multiwavelength images of the CDF-S XT2 neighbourhood.

    a, Merged Chandra 0.5–7-keV image including the entire 7-Ms CDF-S survey. b, Same as a, but excluding ObsID 16453. c, Chandra 0.5–7-keV image from ObsID 16453 alone. The large dashed circle marks the source region (r = 3.5″) for the extraction of light curves and spectra. The X-ray images in a–c are rendered in counts using the same linear scale, with pixel values ranging from 0 to 8. d, HST/CANDELS F160W image. In each panel, the small red circle marks the position and 1σ positional uncertainty of the source with XID7Ms 330 reported in the 7-Ms CDF-S main catalogue17, while the cyan circle denotes that of the source with XID4Ms 256 reported in the 4-Ms CDF-S main catalogue38. The magenta numbers and crosses mark the object numbers of the six closest galaxies to CDF-S XT2 (see Extended Data Table 2) and their positions in the CANDELS F160W DR1 catalogue22, respectively. For clarity, the position of CDF-S XT2 is not annotated; it is slightly (about 0.3″) northeastern to that of XID7Ms 330.

  2. Extended Data Table 1 Light curve of CDF-S XT2 in logarithmic bins
  3. Extended Data Table 2 X-ray spectral fitting results for CDF-S XT2
  4. Extended Data Table 3 Properties of the six closest galaxies to CDF-S XT2

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