Mergers of Magnetized Neutron Stars with Spinning Black Holes: Disruption, Accretion, and Fallback

We investigate the merger of a neutron star in orbit about a spinning black hole in full general relativity with a mass ratio of $5:1$, allowing the star to have an initial magnetization of ${10}^{12}\mathrm{G}$. We present the resulting gravitational waveform and analyze the fallback accretion as the star is disrupted. We see no significant dynamical effects in the simulations or changes in the gravitational waveform resulting from the initial magnetization. We find that only a negligible amount of matter becomes unbound; 99% of the neutron star material has a fallback time of 10 seconds or shorter to reach the region of the central engine and that 99.99% of the star will interact with the central disk and black hole within 3 hours.

Figure 1

An isosurface of density ($6.18\times {10}^{10}\mathrm{g}/{\mathrm{cm}}^3$) and the apparent horizon (black spheroid) at various times for the magnetized evolution.

Figure 2

(left) The $l=2$, $m=2$ mode of $r{\Psi }_4$ for mergers examined here (removing the initial artificial stage related to the initial data). Both the magnetic and nonmagnetic waveforms are the same; extraction performed at coordinate distances 295 km, 332 km, and 369 km and adjusted for travel time. (right) Power spectrum of the wave-strain $f|h|$ in which in grey bands denote frequencies associated with the ISCO and quasinormal ringing corresponding to a black hole with mass $M_T=8.33M_{\odot }$ and spin of $a/M_T=(0.56,0.7)$.

Figure 3

The integrated mass (solid line) vs time for the magnetic case. The plus signs indicate the integral of material only outside the ISCO. The dashed curve shows the integral only over material moving slower than the escape speed for a 7 solar mass black hole. The dotted-dashed line (lowest curve) indicates unbound material (moving at or faster than the escape speed, regardless of direction) multiplied by a factor of 4 to put it on the same scale. The equivalent integrals for the unmagnetized simulation yield nearly identical values up to $t\simeq 20\mathrm{ms}$.

Figure 4

Behavior at $t=22.2\mathrm{ms}$ (axis in kms and density color key in $\mathrm{g}/{\mathrm{cm}}^3$). (a) density isosurface and magnetic field lines with the black hole indicated by the central black spheroid. (b)–(d) fluid density grouped according to the speed of the fluid in the equatorial plane. There is no fluid with velocity more than 0.5.

Figure 5

Fallback accretion histogram. For all bound mass beyond 1.5 times the ISCO, a fallback time is estimated and the results are binned as shown. Distributions are computed for 3 times shown by the different colors that roughly correspond to the start of the tidal disruption event, the peak of gravitational radiation emitted from the system and the point where a disk encircles the central black hole. The dashed line indicates a power law with a slope of $-2/3$ for reference.