Production of Jets before Neutron Star Mergers

Abstract

We demonstrate that magnetospheric interactions between merging neutron stars (NSs) generate dual-jetted current outflows, analogous to the Alfvén wings observed during planetary interactions in the Solar System. Using 3D relativistic MHD simulations, we model the interaction as a conducting sphere moving through a highly magnetized plasma of the companion's magnetosphere. Unusually, the interaction operates in a regime that is relativistic yet sub-Alfvénic. Electromagnetic draping amplifies magnetic fields in a narrow layer near the stellar surface, leading to the generation of electric currents along the local magnetic field. The generation of beamed outflows enhances the instantaneous power of the pulsar-like radio and high-energy emission, produces spin/orbital modulations, and is likely to lead to observable precursor emission preceding the main gravitational wave event.

Figures (16)

• Figure 1: Non-relativistic Sub-Alfvénic run A1. Top row: $xy$-plane slices of flow velocity ($v$), density ($\rho$), ram pressure ($p_{\mathrm{ram}}$), and fast magnetosonic Mach number ($\mathcal{M}_{f}$). Bottom row: corresponding $xz$-plane slices. All quantities, except $\mathcal{M}_{f}$, are normalized by the upstream values. Velocity Streamlines and magnetic field lines are overlaid where applicable. The green contour in fast Mach plots marks the boundary $\mathcal{M}_{f}=1$.
• Figure 2: Non-relativistic Sub-Alfvénic run A1. 3D contour plot of the magnetic pressure ${B^2}/{8\pi}$ at the steady state viewed along the x-axis.
• Figure 3: Alfvén wing structure in illustrative run A1c. (a) Isosurface of current density magnitude $|\mathbf{J}|$ showing bipolar field-aligned current channel (purple) extending from the neutron star surface. The green and red arrows on the wings indicate upward (+z) and downward (-z) directed currents, respectively, forming the bipolar Alfvén wing current system. (b) Current streamlines colored by $J_z$ component, revealing the field-aligned nature of the currents. Both panels show arrows indicating ambient magnetic field $\mathbf{B}_0$ (black) and flow velocity $\mathbf{v}_0$ (dark teal).
• Figure 4: Non-relativistic Sub-Alfvénic run A1. We show 3D streamlines of the current colored by $J_z$ for flow past a perfectly conducting sphere. Left: View along $+z$ direction. The upstream hemisphere ($x>0$) hosts an antisymmetric pair of organized current sheets wrapping around the nose. Downstream ($x<0$), current streamlines show lateral circulation patterns that provide closure paths connecting surface currents to the Alfvén wing channels. Right: View along $+x$ direction. Bipolar, field-aligned $J_z$ structures (Alfvén wings) extend above and below the equator, with organized collimated current channels propagating downstream.
• Figure 5: Non-relativistic Sub-Alfvénic run A1. Here we show 2D slices of parallel current $J_z$ along $x=0$, and $y=0.5$. In the left panel, we observe currents with alternating directions, while an upstream-downstream asymmetry is clearly visible in the right image.