Global features of fast neutrino-flavor conversion in binary neutron star mergers

Binary neutron star merger (BNSM) offers an environment where fast neutrino-flavor conversion (FFC) can vividly occur, which potentially leads to a considerable change of the neutrino radiation field. In this paper, we investigate global features of FFC by general relativistic quantum kinetic neutrino transport simulations in spatial axisymmetry. Our result suggests that global advection of neutrinos plays a crucial role in FFC dynamics. Although flavor conversions occur ubiquitously in the early phase, they remain active only in a narrow region in the late phase. This region includes an ELN-XLN zero surface (EXZS), corresponding to a surface where electron-neutrinos lepton number (ELN) equals the heavy-leptonic one (XLN). The EXZS is not stationary but dynamically evolves in a timescale of global advection. We also find that neutrinos can undergo a flavor swap when they pass through the EXZS, resulting in qualitatively different neutrino radiation fields between both sides of EXZS. Our result suggests that EXZS is one of the key ingredients to characterize FFC in BNSM.

Figure 1

Schematic illustration of neutrino spheres. Neutrinos are radiated at each species-dependent neutrino sphere, which are distinguished by color: red, blue, and green for ${\nu }_e$, ${\overline{\nu }}_e$, and ${\nu }_x$, respectively. At the surface of the hypermassive neutron star denoted by the purple line, we assume all flavors of neutrinos are emitted.

Figure 2

The deviation of number flux ratios from ${\nu }_e$ for a steady-state radiation field obtained from simulations with $H=0$ (Mno model). In the left panel, we show $(|F_{{\overline{\nu }}_e}|/|F_{{\nu }_e}|)-1$, while $(|F_{{\nu }_x}|/|F_{{\nu }_e}|)-1$ is displayed for the right one.

Figure 3

Top: difference of ELN number density from the initial distribution, $n_{\mathrm{ELN}}(t)-n_{\mathrm{ELN}}(0)$. Bottom: ELN-XLN number density, $n_{\mathrm{ELN}\text{−}\mathrm{XLN}}(t)$, in which we highlight its sign by red (for positive) and black (for negative) colors. The boundary between the two regions is a ELN-XLN zero surface; see the text for more details. We displayed them at five different time snapshots for M2h: $t=0.05$, 0.1, 0.2, 0.4, and 0.8 ms from left to right. At $t=0\mathrm{ms}$, the EXZS is located at $\sim 80\mathrm{km}$ along the pole, and it smoothly connects to $\sim 40\mathrm{km}$ at $\theta =55°$.

Figure 4

Color map for the ratio of the real part of the off-diagonal component of $\mathit{f}$ to the sum of the diagonal elements at $t=0.8\mathrm{ms}$ for M2h model.

Figure 5

ELN-XLN angular distributions for M2h. We focus on the region of $\cos {\theta }_{\nu }>0$ (outgoing neutrinos in the radial direction). In top and bottom panels, we show the result at $t=0$ and 0.8 ms, respectively. The left and right panels distinguish the spatial position: $\theta =30°$ and 40° at $r=80\mathrm{km}$.

Figure 6

The difference of neutrino number density between $t=0.8$ and 0 ms, which is normalized by the density at $t=0\mathrm{ms}$. Left: ${\nu }_e$; right: ${\overline{\nu }}_e$. The model is M3h.

Figure 7

Similar to Fig. 6 but for comparisons between the number density and flux, by focusing on ${\nu }_e$ in the M3h model. The left panel corresponds to the same one in Fig. 6, but the range of color contour is different. The right panel displays the result for the norm of neutrino flux.