Neutrino flavor evolution in binary neutron star merger remnants

We study the neutrino flavor evolution in the neutrino-driven wind from a binary neutron star merger remnant consisting of a massive neutron star surrounded by an accretion disk. With the neutrino emission characteristics and the hydrodynamical profile of the remnant consistently extracted from a three-dimensional simulation, we compute the flavor evolution by taking into account neutrino coherent forward scattering off ordinary matter and neutrinos themselves. We employ a “single-trajectory” approach to investigate the dependence of the flavor evolution on the neutrino emission location and angle. We also show that the flavor conversion in the merger remnant can affect the (anti)neutrino absorption rates on free nucleons and may thus impact the $r$-process nucleosynthesis in the wind. We discuss the sensitivity of such results on the change of neutrino emission characteristics, also from different neutron star merger simulations.

Figure 1

Density profile as a function of cylindrical coordinates $z$ and $r_{\mathrm{cyl}}$ at $t=100\mathrm{ms}$ after the merger. The contour $\rho \approx 5\times 10^{11}\mathrm{g}/{\mathrm{cm}}^3$ delimits the innermost part of the disk that is comparable to the surface density of a protoneutron star [35].

Figure 2

Electron fraction (left panel) and matter temperature (right panel) as functions of cylindrical coordinates $z$ and $r_{\mathrm{cyl}}$ at $t=100\mathrm{ms}$ after the merger.

Figure 3

Transport optical depths $\tau$ (color coded) as functions of cylindrical coordinates $z$ and $r_{\mathrm{cyl}}$ at $t=100\mathrm{ms}$ after the merger. The contours (where $\tau \approx 2/3$) correspond to the neutrino transport surfaces associated with ${\nu }_e$ (left), ${\overline{\nu }}_e$ (middle), and ${\nu }_x$ (right).

Figure 4

Accretion disk with a central object (black hole or neutron star) located at the origin. In the case of a black hole, the radius $R_0$ denotes the last stable orbit and defines the inner radius of the disk while for a neutron star, we set $R_0=0$. The (outer) flavor dependent radii of the disk are $R_{{\nu }_{\alpha }}$ for neutrinos and $R_{{\overline{\nu }}_{\alpha }}$ for antineutrinos, respectively. The values used in our emission model are based on detailed simulations of a neutron star merger remnant [35] and are given in Table 2. The polar angle $\theta$ and the azimuthal angle $\varphi$ describe the direction of the neutrino momentum $\mathbf{p}$. Neutrinos are emitted from a point $Q_0$ located in the disk plane at a distance $r_{\mathrm{d}}$ from the origin with a relative angle $\phi$ with respect to the positive $x$ axis and propagate to a point $Q$ located at a distance $x$ from the $z$ axis and a vertical distance $z$ from the equatorial plane.

Figure 5

Geometric factors as a function of the distance from the emission point $x_0=10\mathrm{km}$, $z_0=30\mathrm{km}$. The emission angles correspond to $\theta =20°$ and $\varphi =0°$ while the disk radii are taken from Table 2.

Figure 6

Density profile at 100 ms after the merger as a function of cylindrical coordinates $z$ and $r_{\mathrm{cyl}}$. The neutrino trajectories, shown by the black lines, are specified in Tables 4 and 5. We mirrored the trajectories of Table 4 for clarity. The two reference points $(x_{\mathrm{ref}}^{(i)},z_{\mathrm{ref}}^{(i)})$, $i=1$, 2, are located at a temperature $T\approx 8\mathrm{GK}$ and chosen as representative locations interesting for nucleosynthesis. The points where the matter and unoscillated neutrino self-interaction potentials cancel are marked with ⋆.

Figure 7

Results for selected trajectories specified in Table 4 as functions of the distance for the normal mass hierarchy. The location of the reference point is highlighted by a vertical color band. Top panels: matter potentials ($\lambda$) and absolute values of the unoscillated neutrino potentials ($|\mu |$) along with the vacuum potentials ${\omega }_{21}$ and $|{\omega }_{31}|$ for 5 MeV (anti)neutrinos. The middle panels show the spectral averaged survival probabilities for the electron flavor. The blue curves correspond to ${\nu }_e$ and the red curves to ${\overline{\nu }}_e$, respectively. In the bottom panels, the ratios of capture rates per solid angle for electron neutrinos (blue lines) and antineutrinos (red lines) are presented.

Figure 8

Same as Fig. 7 for the trajectories specified in Table 5.

Figure 9

Flavor evolution results obtained in IH along trajectories 2a (top) and 2d (bottom). The top panels show the spectral averaged survival probabilities for the electron flavor. The blue curves correspond to ${\nu }_e$ and the red ones to ${\overline{\nu }}_e$, respectively. In the bottom panels the ratios of capture rates per solid angle for electron neutrinos (blue curves) and antineutrinos (red curves) are presented.

Figure 10

Neutrino number fluxes [see (27)] (upper panel) and number fluxes times cross sections (lower panel). All quantities are per unit energy per solid angle.

Figure 11

Comparison of the unoscillated potentials between the time snapshots for trajectories 2a (upper panel) and 2c (lower panel).

Figure 12

Electron fraction (left panel) and matter temperature (right panel) as functions of cylindrical coordinates $z$ and $r_{\mathrm{cyl}}$ at $t=60\mathrm{ms}$ after the merger.

Figure 13

Same as Fig. 7 for trajectories 1c (left) and 2a (right) with cooling luminosities (see text).

Figure 14

Ratio of luminosities: $L_{{\nu }_e}/L_{{\nu }_x}$ vs $L_{{\overline{\nu }}_e}/L_{{\nu }_e}$. The corresponding values are given in Table 7. The black point refers to the cooling luminosities of [35] (see text).

Figure 15

Ratio of mean energies: $⟨E_{{\nu }_e}⟩/⟨E_{{\nu }_x}⟩$ vs $⟨E_{{\overline{\nu }}_e}⟩/⟨E_{{\nu }_e}⟩$. The corresponding values are given in Table 7.

Figure 16

Ratio of emission rates: $(L_{{\nu }_e}/E_{{\nu }_e})/(L_{{\nu }_x}/E_{{\nu }_x})$ vs $(L_{{\overline{\nu }}_e}/E_{{\overline{\nu }}_e})/(L_{{\nu }_e}/E_{{\nu }_e})$. The corresponding values are given in Table 7.

Figure 17

Same as Fig. 7 for trajectories 1c and 2a with rescaled fluxes (see text).

Figure 18

Geometry: $Q_0(r_{\mathrm{d}}\cos \phi ,r_{\mathrm{d}}\sin \phi ,0)$, $x=l\sin \vartheta$, $z=l\cos \vartheta$, $\mathrm{d}A=\mathrm{d}{r}_{\mathrm{d}}{r}_{\mathrm{d}}\mathrm{d}\phi$.

Figure 19

Solid angle for each neutrino species computed along trajectory 2a.

Figure 20

Cancellation points (⋆), i.e., points where $\lambda +\mu \approx 0$, for several trajectories above the BNS merger remnant. The emission points are represented by •. Emission and cancellation points with the same color refer to a single trajectory. Points where the crossing starts dominated by matter are excluded. Color coded is the density of matter.