A Magnetar Origin for the Kilonova Ejecta in GW170817

By B. Metzger, T. Thompson, E. Quataert. 2018

165 citations recorded by the source corpus.

The neutron star (NS) merger GW170817 was followed over several days by optical-wavelength (“blue”) kilonova (KN) emission likely powered by the radioactive decay of light r-process nuclei synthesized by ejecta with a low neutron abundance (electron fraction Ye ≈ 0.25–0.35). While the composition and high velocities of the blue KN ejecta are consistent with shock-heated dynamical material, the large quantity is in tension with the results of numerical simulations. We propose an alternative ejecta source: the neutrino-heated, magnetically accelerated wind from the strongly magnetized hypermassive NS (HMNS) remnant. A rapidly spinning HMNS with an ordered surface magnetic field of strength B ≈ (1–3) × 1014 G and lifetime trem ∼ 0.1–1 s can simultaneously explain the velocity, total mass, and electron fraction of the blue KN ejecta. The inferred HMNS lifetime is close to its Alfvén crossing time, suggesting that global magnetic torques could be responsible for bringing the HMNS into solid-body rotation and instigating its gravitational collapse. Different origins for the KN ejecta may be distinguished by their predictions for the emission in the first hours after the merger, when the luminosity is enhanced by heating from internal shocks; the latter are likely generic to any temporally extended ejecta source (e.g., magnetar or accretion disk wind) and are not unique to the emergence of a relativistic jet. The same shocks could mix and homogenize the composition to a low but nonzero lanthanide mass fraction, , as advocated by some authors, but only if the mixing occurs after neutrons are consumed in the r-process on a timescale ≳1 s.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/ee39e3926c66f4737cbd743767bfcfc60d08e9ec

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In Astronomy and Astrophysics

Citations in the corpus

Mass Ejection from the Remnant of a Binary Neutron Star Merger: Viscous-radiation Hydrodynamics Study (2017, background): “For longer timescales, the viscous effect plays a primary role in determining the evolution of the torus surrounding the MNS. Assuming that the torus evolution could be described by a standard accretion disk theory (Shakura & Sunyaev 1973), the viscous accretion timescale is estimated by a a ~ » ´ -”
Binary Neutron Star Mergers: Mass Ejection, Electromagnetic Counterparts, and Nucleosynthesis (2018, background): “…ejected during and shortly after the merger (Chornock et al. 2017; Cowperthwaite et al. 2017; Drout et al. 2017; Nicholl et al. 2017; Perego et al. 2017a; Tanaka et al. 2017; Tanvir et al. 2017; Utsumi et al. 2017; Villar et al. 2017; Waxman et al. 2018; Kawaguchi et al. 2018; Metzger et al. 2018).”
Long-term GRMHD simulations of neutron star merger accretion discs: implications for electromagnetic counterparts (2018, background): “A strongly-magnetized HMNS can produce faster outflows (e.g., Kiuchi et al. 2012; Siegel et al. 2014), resolving this tension (Metzger et al. 2018).”
Constraints on the neutron star equation of state from AT2017gfo using radiative transfer simulations (2018, background): “…that relatively little thermal energy is imparted to the ejecta to power early blue emission in the case of a successful GRB jet] or by internal shocks within whatever variable and temporally extended source (magnetar wind or accretion disc outflow) produces the KN ejecta (Metzger et al. 2018).”
Progenitors of gravitational wave mergers: binary evolution with the stellar grid-based code ComBinE (2018, background): “The final remnant is in most cases a BH, although for double NS mergers a massive NS (or, at least, a meta-stable NS) may be left behind instead depending on the EoS (Vietri & Stella 1998; Falcke & Rezzolla 2014; Metzger et al. 2018).”