GW170817-Like Disk Produces a Blue Kilonova J. M. Miller, with: B. R. Ryan, J. C. Dolence, A. Burrows, C. L. Fryer, O. Korobkin, J. Lippuner, M. R. Mumpower, R. T. Wollaeger

A Groundbreaking Discovery The Case For the Disk Nucleosynthesis

Heat, neutrinos, and magnetic fields can drive a wind off On August 17, 2017, the in-spiral and I I We find structured outflow. I the disk. merger of two neutron stars was observed. I Polar material has high Ye and produces less-robust r-process. This wind may produce a less-robust r-process and be a This event is called GW170817. I I Midplane material has low Ye and produces more-robust r-process. source for the blue kilonova. I Events like this one drive short gamma ray I Structure persists in time! bursts, some of the most energetic events in I Depends on electron fraction Ye. Low Ye means robust the universe. r-process and implies a red kilonova. High means less robust r-process and implies a blue kilonova. I Mergers are sites of r-process Literature is sparse and divided on wind and r-process nucleosynthesis, where the heaviest elements Above: A spectrogram showing the I [3, 4, 5]. in our universe are formed. characteristic gravitational wave chirp due to Above: Volume rendering of a Result depends sensitively on complex interplay of I Many more events to come! GW170817. Image from [1]. I simulated accretion disk formed by a neutrino transport, general relativity, magnetic fields, and BNS merger. fluid dynamics.

An In-Spiral Story Above: Left: Total mass in the outflow as a

4 4 Presenting νbhlight function of time. Right: Average Ye of gravitationally unbound material as a function of 50 50 log column density log column density Neutron stars are ultra-dense remnants of stellar 2 2 I latitude and time. core-collapse, supported against gravity by the strong

y 0 y 0 0 0 nuclear force and neutron degeneracy pressure. I General relativistic magnetohydrodynamics I Natural laboratories for high-density nuclear physics 0.4500 -2 -2 -50 -50 I Accurate fully general-relativistic and frequency 0.3625 I A two solar mass neutron star will have a radius less 4 4 dependent neutrino transport Ye 0.2750 -50 0 50 -50 0 50 than 15km! x x 0.1875 50 50 log column density log column density I Radiation-fluid interaction implemented via 2 2 As stars get close, “tidal tails” of material are 0.1000 I operator splitting thrown off. y 0 y 0 0 0 Capable of solving the post-merger disk problem Eventually the stars form a remnant object I I in full 3D and with all relevant physics. -2 -2 surrounded by a disk of accreting material. This -50 -50 accretion can drive a jet of ultrarelativistic material, Right: Volume rendering of electron fraction in

-50 0 50 -50 0 50 outflow from a ν simulation of a disk Above:Top: Electron fraction of gravitationally Above: Snapshotsx from a simulationx of producing a gamma ray burst. bhlight a binary neutron star merger The tidal formed by a binary-neutron star merger. Red means Above: Relative abundance of yields for disk unbound material at 5 GK vs. latitude. Boxes I The central object may or may not eventually ◦ tails (top right) contain neutron rich a robust r-process. Blue means less robust r-process. outflow: red for material with < 15 off the represent cuts through the data. Red is collapse into a black hole. 2.5 × 103 km ◦ material which is a site of robust midplane and blue for material > 50 . Gray neutron-rich, blue is neutron-poor. Bottom: I Called a Binary1000 Neutron Star (BNS) Merger 4 r-process nucleosynthesis. shading shows the range of values that can be Distribution per solid angle of electron fraction in attained at intermediate angles. material in boxed regions.

log column density 500 I Tidal ejecta eventually forms expanding doughnut of 2 Transport in the Disk material. Electromagnetic Counterpart I This neutron rich material is an ideal site for r-process I Neutrino transport drives Ye down in core and up in corona.

y 0 0 nucleosynthesis of heavy elements. I We perform Monte Carlo radiative transfer to predict what wind-driven kilonova would look like on Early Times: I Neutron star mergers may be the primary source of heavy I Earth. I Neutrinos emitted in core of disk and elements in the universe. -500 I Polar outflow is blue. -2 absorbed in corona. I Radioactive decay of these heavy elements can produce a I Midplane outflow is red. radioactive afterglow, the kilonova. I Late Times: Above: “Expanding doughnut” of More emission than absorption -1000 I -1000 -500 tidal ejecta.0 500 1000 x everywhere Left: Electromagnetic spectra for spherically symmetric outflow composed of nucleosynthetic yields produced in material < 15◦ off the A Surprising Observation midplane, > 50◦ degrees off the midplane, and of solar abundances such as those produced in tidal I Robust r-process in tidal tails produces material opaque to optical light, resulting in red ejecta or outflows like those reported in [4]. At kilonova. 5000A,˚ the polar outflow is ∼ 12× more I The observed kilonova had both red and blue components. The blue component may have luminous than the more neutron-rich outflows. been produced by a site with less robust r-process.

Outlook Above: Rate of emitted neutrinos minus the Above:Lagrangian derivative of electron rate of absorbed neutrinos for electron fraction due to emission or absorption of I We simulate, for the first time, the accretion disk formed by a binary neutron star merger with neutrinos (x < 0) and electron antineutrinos neutrinos: blue for an increase in Ye and red full general relativistic magnetohydrodynamics and neutrino transport. for a decrease. Averaged over azimuthal angle (x > 0). Averaged over azimuthal angle φ and I We find a structured outflow with Ye increasing with angle off of midplane. φ and in time from 0 to 30 ms (left) and from in time from 0 to 30 ms (top) and from 30 ms I We perform nucleosynthesis on disk outflow and find robust r-process in the midplane and less to 127 ms (bottom). 30 ms to 127 ms (right). robust r-process in polar regions. I We model via radiative transfer what the disk-driven kilonova looks like from Earth and show that it is blue. References I We showcase the end-to-end capability of LANL’s multi-disciplinary team.

Phys. Rev. D, 91:124021, Jun 2015. Our work implies that: [1] B. P. Abbott et al. I Gw170817: Observation of gravitational waves from a binary neutron star inspiral. [4] D. M. Siegel and B. D. Metzger. The disk must be included in models of the GW170817 event. Phys. Rev. Lett., 119:161101, Oct 2017. Three-dimensional grmhd simulations of neutrino-cooled accretion disks from neutron star I mergers. [2] D. A. Coulter et al. The Astrophysical Journal, 858(1):52, 2018. I Neutrino transport is critical for modeling this problem. Swope supernova survey 2017a (sss17a), the optical counterpart to a gravitational wave source. Above: Composite optical images from the Swope and Magellan telescope of the GW170817 Science, 358(6370):1556–1558, 2017. [5] R. Fern´andezet al. In future events, a blue kilonova may be produced by the disk from a binary neutron star merger Long-term GRMHD Simulations of Neutron Star Merger Accretion Disks: Implications for I [3] Francois Foucart et al. Electromagnetic Counterparts. Post-merger evolution of a neutron star-black hole binary with neutrino transport. or a black hole neutron star merger. kilonova zero (left) and four (right) days after the merger. The visible spectrum clearly transitions ArXiv e-prints, August 2018. from blue to red [2]. Image compositing by R. Foley. I For more info, see arXiv:1905.07477

This research was funded by the DOE SciDAC program, the DOE LDRD program through CNLS, and the NNSA. This work was approved for unlimited public release The authors would like to thank Charles Gammie and Ben Prather for their insight. as per LA-UR-19-24475