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 $d\sim 30M$ allows the black holes to host large minidisks that suppress the total rest-mass accretion rate variability, which is modulated primarily at $\sim 1.6f_{\mathrm{orb}}$. Each black hole horizon saturates with dimensionless magnetic flux $\varphi \sim 30$. 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.

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• General relativity
• Gravitational wave sources
• Accretion disk & black-hole plasma
• Astronomical black holes
• Astrophysical & cosmological simulations
• Numerical relativity