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

Research output: Contribution to journalArticle

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.
LanguageEnglish
Pages1912-1921
JournalMonthly Notices of the Royal Astronomical Society
Volume483
Early online date15 Nov 2018
StatusPublished - Feb 2019

Cite this

A long-lived neutron star merger remnant in GW170817: constraints and clues from X-ray observations. / Piro, L.; Zhang, Bing; Troja, Eleanora; Ryan, Geoffrey; Van Eerten, Hendrik; Ricci, R; Wieringa, M. H.; Tiengo, A.; Butler, N; Cenko, S. B.; Fox, O. D.; Khandrika, H. G.; Novara, G.; Sakamoto, T.

In: Monthly Notices of the Royal Astronomical Society, Vol. 483, 02.2019, p. 1912-1921.

Research output: Contribution to journalArticle

Piro, L, Zhang, B, Troja, E, Ryan, G, Van Eerten, H, Ricci, R, Wieringa, MH, Tiengo, A, Butler, N, Cenko, SB, Fox, OD, Khandrika, HG, Novara, G & Sakamoto, T 2019, 'A long-lived neutron star merger remnant in GW170817: constraints and clues from X-ray observations' Monthly Notices of the Royal Astronomical Society, vol. 483, pp. 1912-1921.
Piro, L. ; Zhang, Bing ; Troja, Eleanora ; Ryan, Geoffrey ; Van Eerten, Hendrik ; Ricci, R ; Wieringa, M. H. ; Tiengo, A. ; Butler, N ; Cenko, S. B. ; Fox, O. D. ; Khandrika, H. G. ; Novara, G. ; Sakamoto, T. / A long-lived neutron star merger remnant in GW170817: constraints and clues from X-ray observations. In: Monthly Notices of the Royal Astronomical Society. 2019 ; Vol. 483. pp. 1912-1921.
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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.",
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