Magnetized Neutron-Star Mergers and Gravitational-Wave Signals

We investigate the influence of magnetic fields upon the dynamics of, and resulting gravitational waves from, a binary neutron-star merger in full general relativity coupled to ideal magnetohydrodynamics. We consider two merger scenarios: one where the stars have aligned poloidal magnetic fields and one without. Both mergers result in a strongly differentially rotating object. In comparison to the nonmagnetized scenario, the aligned magnetic fields delay the full merger of the stars. During and after merger we observe phenomena driven by the magnetic field, including Kelvin-Helmholtz instabilities in shear layers, winding of the field lines, and transition from poloidal to toroidal magnetic fields. These effects not only mediate the production of electromagnetic radiation, but also can have a strong influence on the gravitational waves. Thus, there are promising prospects for studying such systems with both types of waves.

Figure 1

Fluid density isopycnic (at $\rho =6\times {10}^{13}\mathrm{g}/{\mathrm{cm}}^3$) and magnetic field distribution (in a plane slightly above the equator) at $t=4.4\mathrm{ms}$. Kevin-Helmholtz instability in the shear layer has produced circulation cells that amplify the toroidal magnetic field. The cavities at both trailing edges are due to magnetic pressure inducing buoyancy.

Figure 2

Equatorial axis ratio (semiminor/semimajor) vs time for the nonmagnetized (dashed line) and magnetized (solid line) measured at a density of $1.24\times {10}^{14}\mathrm{g}/{\mathrm{cm}}^3$. The results corresponding to the nonmagnetized case have been shifted in time by $t=3.5\mathrm{ms}$ so both mergers coincide. Notice how the magnetized case approaches the unit value much quicker while the nonmagnetized remains about $\simeq 0.78$. This translates into stronger amplitude in the produced gravitational waves.

Figure 3

Equatorial density contours and velocity field at 2 times for both the nonmagnetized (left column) and magnetized cases (right column). The top frames at $t=4.9\mathrm{ms}$ clearly show the delayed merger caused by magnetic field repulsion. The bottom frames are at $t=18.6\mathrm{ms}$. With no magnetic field, the final differentially rotating star displays a more barlike structure ($m=2$), while the magnetized star is considerably more spherical with a slight offset.

Figure 4

Gravitational-wave signal $h_{+}$ ($l=2$, $m=2$ mode) for the magnetized (solid line) and unmagnetized (dashed) mergers. Before $t\simeq 7.5\mathrm{ms}$, both signals are essentially the same. After $t\simeq 7\mathrm{ms}$, however, the delayed merger of the magnetized stars results in a much stronger signal lasting until $t\simeq 12\mathrm{ms}$. After merger, the redistribution of angular momentum by the magnetic field results in a more axisymmetric remnant whose core rotates more slowly. As a result, at late times the nonmagnetized merger produces radiation with larger (higher) amplitude (frequency) than its magnetized counterpart.