TY - JOUR
T1 - A Magnetar Origin for the Kilonova Ejecta in GW170817
AU - Metzger, Brian D.
AU - Thompson, Todd A.
AU - Quataert, Eliot
N1 - Publisher Copyright:
© 2018. The American Astronomical Society. All rights reserved..
PY - 2018/4/1
Y1 - 2018/4/1
N2 - 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 Y e ≈ 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 t rem ∼ 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.
AB - 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 Y e ≈ 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 t rem ∼ 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.
UR - http://www.scopus.com/inward/record.url?scp=85045548205&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85045548205&partnerID=8YFLogxK
U2 - 10.3847/1538-4357/aab095
DO - 10.3847/1538-4357/aab095
M3 - Article
AN - SCOPUS:85045548205
SN - 0004-637X
VL - 856
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 2
M1 - 101
ER -