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We study an accretion flow during the gravitational-wave driven evolution ofbinary massive black holes. After the binary orbit decays due to an interactionwith a massive circumbinary disk, the binary is decoupled from the circumbinarydisk because the orbital-decay timescale due to emission of gravitational wavebecomes shorter than the viscous timescale evaluated at the inner edge ofcircumbinary disk. During the subsequent evolution, the accretion disk, whichis truncated at the tidal radius because of the tidal torque, also shrinks asthe orbital decay. Assuming that the disk mass changed by this process is allaccreted, the disk becomes radiatively inefficient when the semi-major axis isseveral hundred Schwarzschild radii. The high-energy radiations, in spite of alow bolometric luminosity, are emitted from an accretion disk around each blackhole long before the black hole coalescence as well as the gravitational wavesignals. The synchrotron process can notably produce potentially observableradio emissions at large distances if there is a strong, dipole magnetic fieldaround each black hole. In unequal mass-ratio binaries, step-like lightvariations are seen in the observed light curve because the luminosity ishigher and its duration time are shorter in the radio emission by the diskaround the secondary black hole than those of the primary black hole. Such aprecursor would be unique to not a single black hole system but a binary blackhole system, and implies that binary black holes finally merge withoutaccretion disks.

Radiatively inefficient accretion flows induced by gravitational-wave emission before massive black hole coalescence