Neutrino transport in general relativistic neutron star merger simulations

Abstract

Numerical simulations of neutron star-neutron star and neutron star-black hole binaries play an important role in our ability to model gravitational-wave and electromagnetic signals powered by these systems. These simulations have to take into account a wide range of physical processes including general relativity, magnetohydrodynamics, and neutrino radiation transport. The latter is particularly important in order to understand the properties of the matter ejected by many mergers, the optical/infrared signals powered by nuclear reactions in the ejecta, and the contribution of that ejecta to astrophysical nucleosynthesis. However, accurate evolutions of the neutrino transport equations that include all relevant physical processes remain beyond our current reach. In this review, I will discuss the current state of neutrino modeling in general relativistic simulations of neutron star mergers and of their post-merger remnants. I will focus on the three main types of algorithms used in simulations so far: leakage, moments, and Monte-Carlo scheme. I will review the advantages and limitations of each scheme, as well as the various neutrino-matter interactions that should be included in simulations. We will see that the quality of the treatment of neutrinos in merger simulations has greatly increased over the last decade, but also that many potentially important interactions remain difficult to take into account in simulations (pair annihilation, oscillations, inelastic scattering).

Keywords: Neutrino radiation transport; Neutron star merger simulations; Numerical relativity methods.

Fig. 1

Merger of a disrupting NSBH binary (Right) and of a low-mass NSNS binary (Left). In disrupting NSBH systems, most of the matter is rapidly accreted onto the black hole, while the rest forms an accretion disk and extended tidal tail. Low-mass NSNS binaries form a massive neutron star remnant surrounded by a bound disk, with a smaller amount of material ejected in the tidal tail. The right panel is reproduced with permission from Foucart et al. (2017), copyright by IOP; the left panel visualizes a simulation from Foucart et al. (2016a)

Fig. 2

Post-merger remnant a few milliseconds after a BNS merger (Left), and 0.3 s after a NSBH merger (Right). The BNS system forms a massive, differentially rotating neutron star surrounded by a low-mass accretion disk, with shocked spiral arms visible in the disk. The NSBH system forms an extended accretion disk around the remnant black hole, with collimated magnetic fields in the polar region. The right panel is reproduced with permission from Hayashi et al. (2022a), copyright by APS; the left panel visualizes a simulation from Foucart et al. (2016a)

Fig. 3

Two neutrino beams crossing in a two-moment simulation using the Minerbo closure (Left), and in a two-moment simulation in which the pressure tensor is taken from the result of a Monte-Carlo evolution of the transport equations (Right). We see that the Minerbo closure leads to an artificial collision between the beams. Image reproduced with permission from Foucart (2018), copyright by the authors

Fig. 4

Advection of trapped neutrinos in a rapidly moving fluid ($v=0.5c$) with high scattering opacity, and no absorption or emission. The different simulations use (i) the latest implementation of the Jacobian matrix and high-order numerical fluxes from Radice et al. (2022) (blue), (ii) approximate sources and a diffusion equation (ZelmaniM1 code, black), or (iii) only first order in v/c terms (green). Image reproduced with permission from Radice et al. (2022), copyright by the authors

Fig. 5

Density, velocity, and heating rate from neutrino absorption 20 ms after a neutron star merger in a simulation using pair annihilation (Left), and a similar simulation ignoring that effect (Right). Note the significant change in the velocity of the outflows, which persists up to the end of these simulations (300 ms post-merger). Image reproduced with permission from Fujibayashi et al. (2017), copyright by AAS

Fig. 6

Total neutrino luminosity from the neutron star (solid curves) and accretion disks (dashed curve) in three long simulations of NSNS merger remnants in which the central object remains a neutron star. We observe both the decay of the disk emission on O(100 ms) timescales and the sustained emission from the neutron star Image reproduced with permission from Fujibayashi et al. (2020), copyright by AAS

Fig. 7

Electron fraction in post-merger remnants. We show a vertical slice through a NSBH merger (Left) and a BNS merger (Right). The BNS merger is evolved using two different choices for the energy closure in a two-moment scheme. Images reproduced with permission from Foucart et al. (2015) and Foucart et al. (2016b), copyright by APS

Fig. 8

Neutrino luminosities for ${\nu }_e$, ${\overline{\nu }}_e$ and ${\nu }_x$ in a BNS merger with a long-lived remnant. Results are shown for three algorithms: a hybrid leakage-moment scheme (blue), a two-moment scheme with Minerbo closure (orange), and a two-moment scheme with Eddington closure (green). Image reproduced with permission from Radice et al. (2022), copyright by the authors

Fig. 9

Comparison of the electron fraction $Y_e$ in a vertical slice of a post-merger remnant for simulations using Monte-Carlo methods (Left) and a two-moment scheme (Middle). The right panel shows the $Y_e$ distribution of matter outflows in both simulations. Image reproduced with permission from Foucart et al. (2020), copyright by AAS

Fig. 10

Left: Energy spectrum of the ${\nu }_x$ neutrinos 14 ms after a BNS merger, according to a Monte-Carlo simulation. Vertical lines show the average energy esimated using Monte-Carlo (Solid line) or a two-moment scheme (dashed line). Right: Angular distribution of the neutrinos in the same simulations, using a Monte-Carlo evolution (black) or a two-moment evolution (red). Image reproduced with permission from Foucart et al. (2018), copyright by APS