Binary Black-Hole Mergers in Magnetized Disks: Simulations in Full General Relativity

We present results from the first fully general relativistic, magnetohydrodynamic (MHD) simulations of an equal-mass black-hole binary (BHBH) in a magnetized, circumbinary accretion disk. We simulate both the pre- and postdecoupling phases of a BHBH-disk system and both “cooling” and “no-cooling” gas flows. Prior to decoupling, the competition between the binary tidal torques and the effective viscous torques due to MHD turbulence depletes the disk interior to the binary orbit. However, it also induces a two-stream accretion flow and mildly relativistic polar outflows from the BHs. Following decoupling, but before gas fills the low-density “hollow” surrounding the remnant, the accretion rate is reduced, while there is a prompt electromagnetic luminosity enhancement following merger due to shock heating and accretion onto the spinning BH remnant. This investigation, though preliminary, previews more detailed general relativistic, MHD simulations we plan to perform in anticipation of future, simultaneous detections of gravitational and electromagnetic radiation from a merging BHBH-disk system.

Figure 1

The disk $\alpha$ parameter (upper panel) and surface-density $\Sigma$ (lower panel) profiles. ${\Sigma }_0$ is the maximum surface density at $t=0$. Dotted magenta line is the initial data, solid lines are at decoupling, and dashed lines are at merger. Black lines are from the no-cooling case, and light gray (green) lines are from the cooling case.

Figure 2

Orbital plane snapshot of rest-mass density $\log ({\rho }_0/{\rho }_{0,\max })$ from the no-cooling simulation at $t\sim 10000M$ in the relaxed disk, prior to decoupling. The inset zooms in on the region close to the BHs.

Figure 3

Time-averaged binary accretion rate ${\dot{M}}_{\mathrm{BHBH}}$, normalized to the average value for a single BH ${\dot{M}}_{\mathrm{BH}}$, versus time. Colors have the same meaning as in Fig. 1. Left-hand panel: Predecoupling ($a=\mathrm{const}$) phase. Right-hand panel: Postdecoupling (inspiral) phase. ${\dot{M}}_{\mathrm{BH}}=0.45n_{12}{M}_8^2{M}_{\odot }{\mathrm{yr}}^{-1}$, where $M_8\equiv M/{10}^8{M}_{\odot }$ and $n_{12}\equiv n/{10}^{12}{\mathrm{cm}}^{-3}$ is the initial maximum gas particle number density. Merger occurs at time $t_m=11743M$.

Figure 4

Meridional snapshot of magnetic pressure $\log (P_M/{\rho }_{0,\max })$ and fluid velocity vectors at $t\sim 10000M$ for the no-cooling case.

Figure 5

The Poynting luminosity $L_{\mathrm{EM}}$ measured at $R=100M$ as a function of time [light gray (green) line for cooling, black line for no-cooling]. The cooling luminosity $L_{\mathrm{cool}}$ from the cooling case [gray (magenta) line]. ${\dot{M}}_{\mathrm{BH}}{c}^2=2.58\times {10}^{46}\mathrm{erg}{\mathrm{s}}^{-1}{n}_{12}{M}_8^2$.