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In the $\Lambda$CDM scenario, small galaxies merge to produce largerentities. Since supermassive black holes (SMBHs) are found in galaxies of allsizes, SMBH binaries (SMBHBs) are generally expected to form during theamalgamation of galaxies. It is unclear what fraction of these binaries couldeventually merge, but a general consensus is that initially the orbital decayis mediated by the surrounding gas and stars. In this Letter, we show that inactive galactic nulcei (AGNs) the radiation field also causes the orbits of theaccreting SMBHs to shrink. The corresponding mechanism, known as the"Poynting-Robertson drag" (PR drag), takes effect on a well-defined timescale$CT_{\rm Sal}$, where $T_{\rm Sal}$ is the Salpeter timescale of the AGN,presumably coincide with the primary SMBH, and$C=4\xi^{-1}\epsilon^{-1}q^{1/3}(1+q)^{2/3}(1-\epsilon)$ is a constantdetermined by the radiative efficiency $\epsilon$, the mass ratio $q$ of thetwo black holes, and a parameter $\xi$ characterizing the size of thecircum-secondary accretion disk. We find that when $q\lesssim$afew$\times10^{-5}$, the PR drag is more efficient in shrinking the binary thanmany other mechanisms, such as dynamical friction and type-I migration. Ourfinding points to a possible new channel for the coalescence of unequal SMBHBsand the clearing of intermediate-massive black holes in AGNs.

A Shining Death of Unequal Supermassive Black Hole Binaries