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

Y. Q. Xue (), X. C. Zheng (), Y. Li, W. N. Brandt, B. Zhang (), B. Luo, B.-B. Zhang, F. E. Bauer, H. Sun, B. D. Lehmer, X.-F. Wu, G. Yang, X. Kong, J. Y. Li, M. Y. Sun, J.-X. Wang and F. Vito
Additional contact information
Y. Q. Xue: University of Science and Technology of China
X. C. Zheng: University of Science and Technology of China
Y. Li: Peking University
W. N. Brandt: The Pennsylvania State University
B. Zhang: University of Nevada
B. Luo: Nanjing University
B.-B. Zhang: Nanjing University
F. E. Bauer: Instituto de Astrofísica and Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile
H. Sun: Chinese Academy of Sciences
B. D. Lehmer: University of Arkansas
X.-F. Wu: University of Science and Technology of China
G. Yang: The Pennsylvania State University
X. Kong: University of Science and Technology of China
J. Y. Li: University of Science and Technology of China
M. Y. Sun: University of Science and Technology of China
J.-X. Wang: University of Science and Technology of China
F. Vito: Instituto de Astrofísica and Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile

Nature, 2019, vol. 568, issue 7751, 198-201

Abstract: Abstract Mergers of neutron stars are known to be associated with short γ-ray bursts1–4. 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 field5–8 (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.

Date: 2019
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DOI: 10.1038/s41586-019-1079-5

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