Full transport model of GW170817-like disk produces a blue kilonova

The 2017 detection of the inspiral and merger of two neutron stars in gravitational waves and gamma rays was accompanied by a quickly reddening transient. Such a transient was predicted to occur following a rapid neutron capture (r-process) nucleosynthesis event, which synthesizes neutron-rich, radioactive nuclei and can take place in both dynamical ejecta and in the wind driven off the accretion torus formed after a neutron star merger. We present the first three-dimensional general relativistic, full transport neutrino radiation magnetohydrodynamics simulations of the black hole-accretion disk-wind system produced by the GW170817 merger. We show that the small but non-negligible optical depths lead to neutrino transport globally coupling the disk electron fraction, which we capture by solving the transport equation with a Monte Carlo method. The resulting absorption drives up the electron fraction in a structured, continuous outflow, with electron fraction as high as $Y_e\sim 0.4$ in the extreme polar region. We show via nuclear reaction network and radiative transfer calculations that nucleosynthesis in the disk wind will produce a blue kilonova.

Figure 1

Volume rendering of the electron fraction $Y_e$ of material in the disk-wind system after $\sim 31.7\mathrm{ms}$. Opacity in image is proportional to temperature.

Figure 2

Top: electron fraction of gravitationally unbound material at 5 GK vs latitude, $|90-{\theta }_{\mathrm{bl}}|$. Boxes represent cuts through the data. Red is neutron rich, blue is neutron poor. Black dashed lines represent approximate bounds on viewing angle for gw170817, as given by [65]. (Although angle matters, an observation integrates over many lines of sight.) Bottom: distribution per solid angle of electron fraction in material in boxed regions.

Figure 3

Left: total mass in the outflow as a function of time. Right: average electron fraction $Y_e$ of gravitationally unbound material at an extraction radius of $r\sim {10}^3\mathrm{km}$ as a function of latitude and time.

Figure 4

Relative abundance of yields for disk outflow: red for material with $<15°$ off the midplane and blue for material $>50°$. Gray shading shows the range of values that can be attained at intermediate angles. Black dashed line shows yields attained in the GW170817 box in Fig. 2. Curves are normalized by mass fraction. Solar abundances from [76] shown in green.

Figure 5

Rate of emitted neutrinos minus the rate of absorbed neutrinos for electron neutrinos ($x<0$) and electron antineutrinos ($x>0$). Averaged over azimuthal angle $\varphi$ and in time from 0 to 30 ms (top) and from 30 ms to 127 ms (bottom).

Figure 6

Lagrangian derivative of electron fraction due to emission or absorption of neutrinos: blue for an increase in $Y_e$ and red for a decrease. Averaged over azimuthal angle $\varphi$ and in time from 0 to 30 ms (left) and from 30 ms to 127 ms (right). Red curves define a surface at which gravitationally unbound material reaches within 5% of its asymptotic $Y_e$ at infinity, roughly indicating where neutrino interactions significantly effect the electron fraction of escaping material. Very little material becomes unbound closer to the black hole than the innermost radius of the red curves.

Figure 7

Electromagnetic spectra for spherically symmetric outflow composed of nucleosynthetic yields produced in material $<15°$ off the midplane, $>50°$ degrees off the midplane, and of solar abundances such as those produced in tidal ejecta or outflows like those reported in [52]. At 5000 Å, the polar outflow is $\sim 12\times$ more luminous than the more neutron-rich outflows.

Figure 8

$\varphi$-averaged quality factor for the MRI in the midplane for three different times. Blue shows the quality factor for lab-frame vertical component. Red shows it for the comoving magnetic field.