Spherical symmetry in the kilonova AT2017gfo/GW170817

Abstract

The mergers of neutron stars expel a heavy-element enriched fireball which can be observed as a kilonova. The kilonova's geometry is a key diagnostic of the merger and is dictated by the properties of ultra-dense matter and the energetics of the collapse to a black hole. Current hydrodynamical merger models typically show aspherical ejecta. Previously, Sr${}^{+}$ was identified in the spectrum of the the only well-studied kilonova AT2017gfo, associated with the gravitational wave event GW170817. Here we combine the strong Sr${}^{+}$ P Cygni absorption-emission spectral feature and the blackbody nature of kilonova spectrum, to determine that the kilonova is highly spherical at early epochs. Line shape analysis combined with the known inclination angle of the source also shows the same sphericity independently. We conclude that energy injection by radioactive decay is insufficient to make the ejecta spherical. A magnetar wind or jet from the black hole disk could inject enough energy to induce a more spherical distribution in the overall ejecta, however an additional process seems necessary to make the element distribution uniform

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• Measuring the Hubble constant with kilonovae using the Expanding Photosphere Method
• Emergence hour-by-hour of $r$-process features in the kilonova AT2017gfo
• Nucleosynthesis in outflows of compact objects and detection prospects of associated kilonovae
• Neutrinos from explosive transients at the dawn of multi-messenger astronomy
• Differentially rotating scalarized neutron stars with realistic post-merger profile
• Cerium features in kilonova near-infrared spectra: implication from a chemically peculiar star
• Impact of jets on kilonova photometric and polarimetric emission from binary neutron star mergers
• Kilonova evolution -- the rapid emergence of spectral features
• Accretion spin-up of the massive component in the neutron star stripping model for short gamma-ray bursts
• Helium features are inconsistent with the spectral evolution of the kilonova AT2017gfo
• Interpolated kilonova spectra models: necessity for a phenomenological, blue component in the fitting of AT2017gfo spectra
• R-process heating implementation in hydrodynamic simulations with neural networks
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• What to expect: kilonova light curve predictions via equation of state marginalization
• Light Travel Time Effects in Kilonova Models
• Inferring kilonova ejecta photospheric properties from early blackbody spectra
• Non-conservative Mass Transfer in the Neutron Star Stripping Model