Binary black holes gone MAD: Magnetically arrested minidisks around nonspinning black holes

TL;DR

This work demonstrates that magnetically arrested minidisks can form around equal-mass, nonspinning BBHs accreting from a circumbinary disk in full 3+1 GR. Using high-resolution GRMHD simulations with d/M=30, the minidisks saturate their horizon magnetic flux at φ_BH ≈ 25–30 and exhibit recurrent magnetic-flux eruptions driven by reconnection, producing flares and modulated horizon-scale dynamics. The total rest-mass accretion rate remains comparatively steady due to alternating accretion onto the two BHs, while spectral power shows peaks near 1.6 f_orb for the total accretion and near 0.8 f_orb for individual BHs, indicating complex but detectable variability. These results establish MAMs as a new outcome of CBD accretion and suggest that electromagnetic counterparts to gravitational waves from BBHs could arise from horizon-scale magnetic activity, warranting future GRRT/radiative-transfer studies for observable predictions.

Abstract

We demonstrate the formation of magnetically arrested minidisks (MAM) around equal-mass, nonspinning binary black holes with magnetohydrodynamic simulations of circumbinary disk accretion in full 3+1 general relativity. The initial separation of $d\sim 30\,M$ allows the black holes to host large minidisks that suppress the total rest-mass accretion rate variability, which is modulated primarily at $\sim 1.6 \, f_{\rm orb}$. Each black hole horizon saturates with dimensionless magnetic flux $φ\sim 30$. Magnetic reconnection near the horizons drives recurrent eruptions which are expected to drive flaring in the infrared and X-ray bands. Our results establish MAMs as a new outcome of circumbinary disk accretion, and a promising source of novel electromagnetic counterparts to gravitational waves from binary black holes.

Figures (3)

• Figure 1: Top: Rest-mass accretion rate $\dot{M}$ onto each BH (orange and blue lines) and the sum of the two (solid black line) normalized by the total time averaged $\langle \dot{M} \rangle$ for the time period $15 \lesssim t/10^3 M \lesssim 22$. Bottom: Dimensionless magnetic flux $\phi$ on each BH (orange and blue lines) and the sum of the two (solid black line). We also include the standard deviation of each timeseries. The vertical translucent gray lines indicate times plotted in Figure \ref{['fig:single']}. Right: Normalized power spectral density (PSD) of the Fourier transforms of the signals in the left column. We normalize the frequency axis by the binary's orbital frequency and normalize the amplitudes by the maximum of the three PSDs shown. The individual signals all show a peak periodicity at $f\sim 0.8 \, f_{\rm orb}$, whereas the total signals show a suppressed variability with the $\dot{M}$ peaking at $f\sim 1.6 \text{ and } 1.0 f_{\rm orb}$, while $\phi_{\rm BH}$ peaks at $f\sim 1 \text{ and } 1.5 \, f_{\rm orb}$. We have checked that the Fourier transform of the unnormalized horizon flux is the same.
• Figure 2: We plot the rest-mass density and magnetic field structure at horizon-scales at a pre-flare quiescent state (left panel), during an active flaring state (center panel), and at a post-flare quiescent state (right panel). The rest-mass density $\rho_0$ is normalized by its initial maximum in the disk $\rho_{0, \rm max}$ on a color scale where brighter colors indicate higher densities. We overplot the magnetic field with directed streamlines and indicate the BH horizon with a black disk. In the left panel, accretion proceeds through an equatorial flow that is slightly displaced from the binary orbital plane. In the center panel, there is an active flux eruption to the left of the BH, where the vertical magnetic field lines are ordered, thread the binary orbital plane, and extend vertically, and the density has dropped to $\rho_0/\rho_{0, \rm max}\sim 10^{-2}$. In the right panel, the flux bundle is at $\bar{r} \sim6M$ on the right hand side of the minidisk and shares the same magnetic field characteristics as in the middle panel.
• Figure 3: Top two rows: slices of the rest-mass density ($\rho_0$) normalized by its initial maximum ($\rho_{0, \rm max}$) on a color scale where brighter colors indicate higher densities. Bottom two rows: slices of the plasma beta ($\beta \equiv P_{\rm gas}/P_{\rm mag}$) on a color scale where white indicates equipartition and red (blue) indicates that magnetic (gas) pressure dominates. We overplot magnetic field lines in white and indicate the BH horizon with a black disk. First and third rows: we plot the system in the orbital plane of the binary. Second and fourth rows: we plot the system in the $\bar{r}-z$ plane, with $\bar{r}$ centered around the BH and going through a magnetic flux bundle. The first column plots the system when the horizon magnetic flux is saturated before an eruption event. The following three columns show the magnetic flux bundle erupting from the horizon, moving through the minidisk, and entering the cavity.