A long-lived neutron star merger remnant in GW170817: constraints and clues from X-ray observations

L. Piro, Bing Zhang, Eleanora Troja, Geoffrey Ryan, Hendrik Van Eerten, R Ricci, M. H. Wieringa, A. Tiengo, N Butler, S. B. Cenko, O. D. Fox, H. G. Khandrika, G. Novara, T. Sakamoto

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Abstract

Multimessenger observations of GW170817 have not conclusively established whether the merger remnant is a black hole (BH) or a neutron star (NS). We show that a long-lived magnetized NS with a poloidal field B ≈ 1012 G is fully consistent with the electromagnetic dataset, when spin-down losses are dominated by gravitational wave (GW) emission. The required ellipticity ε ≳ 10−5 can result from a toroidal magnetic field component much stronger than the poloidal component, a configuration expected from an NS newly formed from a merger. Abrupt magnetic dissipation of the toroidal component can lead to the appearance of X-ray flares, analogous to the one observed in gamma-ray burst (GRB) afterglows. In the X-ray afterglow of GW170817, we identify a low-significance (≳3σ) temporal feature at 155 d, consistent with a sudden reactivation of the central NS. Energy injection from the NS spin-down into the relativistic shock is negligible, and the underlying continuum is fully accounted for by a structured jet seen off-axis. Whereas radio and optical observations probe the interaction of this jet with the surrounding medium, observations at X-ray wavelengths, performed with adequate sampling, open a privileged window on to the merger remnant.
Original languageEnglish
Pages (from-to)1912-1921
Number of pages10
JournalMonthly Notices of the Royal Astronomical Society
Volume483
Issue number2
Early online date15 Nov 2018
DOIs
Publication statusPublished - 1 Feb 2019

Keywords

  • Gamma
  • Gravitational waves
  • Ray burst: general
  • Stars: neutron

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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