Many astrophysical binaries, from planets to black holes (BHs), exert strong torques on their circumbinary accretion disks, and are expected to significantly modify the disk structure. In this talk I will discuss a distinct regime, where the tidal barrier of the secondary causes a significant pile-up of gas outside of its orbit, which can lead to the closing of the gap. If the secondary is less massive than ~10^6 Msun, then the gap is closed before gravitational waves (GWs) start dominating the orbital decay. In this regime, the disk is still strongly perturbed, but the piled-up gas continuously overflows as in a porous dam, and crosses inside the secondary's orbit. The corresponding migration rate, which we label Type 1.5, is slower than the usual limiting cases known as Type I and II migration. The overflowing disk becomes several hundred times brighter in the optical bands than in AGNs with a solitary BH. Surveys such as PanSTARRS or LSST may discover the periodic variability of this population of binaries, and constrain the expected rates for future GW detections with LISA.
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