Post-merger evolution of a neutron star-black hole binary with neutrino transport

Abstract

We present a first simulation of the post-merger evolution of a black hole-neutron star binary in full general relativity using an energy-integrated general relativistic truncated moment formalism for neutrino transport. We describe our implementation of the moment formalism and important tests of our code, before studying the formation phase of an accretion disk after a black hole-neutron star merger. We use as initial data an existing general relativistic simulation of the merger of a neutron star of mass 1.4M with a black hole of mass 7M and dimensionless spin χBH = 0.8. Comparing with a simpler leakage scheme for the treatment of the neutrinos, we find noticeable differences in the neutron to proton ratio in and around the disk, and in the neutrino luminosity. We find that the electron neutrino luminosity is much lower in the transport simulations, and that both the disk and the disk outflows are less neutron-rich. The spatial distribution of the neutrinos is significantly affected by relativistic effects, due to large velocities and curvature in the regions of strongest emission. Over the short timescale evolved, we do not observe purely neutrino-driven outflows. However, a small amount of material (3 × 10 −4 M ) is ejected in the polar region during the circularization of the disk. Most of that material is ejected early in the formation of the disk, and is fairly neutron rich (electron fraction Ye ∼ 0.15 − 0.25). Through r-process nucleosynthesis, that material should produce high-opacity lanthanides in the polar region, and could thus affect the lightcurve of radioactively powered electromagnetic transients. We also show that by the end of the simulation, while the bulk of the disk remains neutron-rich (Ye ∼ 0.15 − 0.2 and decreasing), its outer layers have a higher electron fraction: 10% of the remaining mass has Ye > 0.3. As that material would be the first to be unbound by disk outflows on longer timescales, and as composition evolution is slower at later times, the changes in Ye experienced during the formation phase of the disk could have an impact on nucleosynthesis outputs from neutrino-driven and viscously-driven outflows. Finally, we find that the effective viscosity due to momentum transport by neutrinos is unlikely to have a strong effect on the growth of the magnetorotational instability in the post-merger accretion disk.

The role of weak interactions in dynamic ejecta from binary neutron star mergers

“…Furthermore, GR radiation-hydrodynamics simulations also found that Y e in the dynamic ejecta of BNS mergers can be significantly enhanced by weak reactions (e.g., Sekiguchi et al 2015, Foucart et al 2015). However, Sekiguchi et al (2015) reported that the inclusion of neutrino absorption in optically thin conditions has a minor impact on the average electron fraction of the ejecta in their models.…”

“…However, weak reactions at high temperatures and densities are expected to change the relative amount of neutrons and protons, as a results of an asymmetric behavior of electron neutrinos and antineutrinos. In particular, Lν e > L νe (Eichler et al 1989, Ruffert et al 1997, Rosswog & Liebendörfer 2003 and the formation of an excess ofν e deep inside the hot remnant (Foucart et al 2015) increase the proton and electron abundances. To take these effects into account, we distinguish between different regimes (also summarized in Figure 2) to initialize Y e :…”

“…Numerical simulations of BNS mergers, including neutrino emission, show characteristic trends in the evolution of the neutrino luminosities, L ν , and mean energies, E ν (Ruffert et al 1997, Rosswog & Liebendörfer 2003, Neilsen et al 2014, Foucart et al 2015, Sekiguchi et al 2015. While the emission of radiation is negligible during the cold inspiral phase, as soon as the two NSs touch (t = t m ) and the temperature in the remnant increases, neutrino luminosities are boosted and reach their peak values (a few 10 53 erg s −1 ) within a few ms (t = t peak ).…”

“…Recent GR simulations, including microphysical equation of state (EOS) and neutrino treatment, indicate that the electron fraction of the dynamic ejecta can be significantly changed with respect to the initial cold weak equilibrium values (Wanajo et al 2014, Sekiguchi et al 2015, Foucart et al 2015, Goriely et al 2015. Matter ejected by tidal interaction is expected to stay relatively cold.…”

“…In current simulations, more microphysics has been included, namely high density equations of state (e.g., Hotokezaka et al 2011, Bauswein et al 2013, Read et al 2013, Rezzolla & Takami 2016, Bernuzzi et al 2016, Radice et al 2017) and neutrinos (e.g., Neilsen et al 2014, Foucart et al 2015, Sekiguchi et al 2015. The transport of neutrinos produced in the hot and dense remnant is not full consistently included in current 3D, full general relativistic (GR) simulations.…”