Changing-look inspirals: trends and switches in AGN disc emission as signposts for merging black hole binaries

ABSTRACT Using grid-based hydrodynamics simulations and analytic modeling, we compute the electromagnetic (EM) signatures of gravitational wave (GW) driven inspirals of massive black hole binaries that accrete gas from circumbinary discs, exploring the effects of varying gas temperatures, viscosity laws, and binary mass ratios. Our main finding is that active galactic nuclei (AGNs) that host inspiraling binaries can exhibit two sub-types of long-term secular variability patterns: Type-A events which dim before merger and brighten afterward, and Type-B events which brighten before merger and dim afterward. In both types, the merger coincides with a long-lasting chromatic change of the AGN appearance. The sub-types correspond to the direction of angular momentum transfer between the binary and the disc, and could thus have correlated GW signatures if the gas-induced torque can be inferred from GW phase drift measurements by LISA. The long-term brightness trends are caused by steady weakening of the disc-binary torque that accompanies orbital decay, it induces a hysteresis effect whereby the disc ‘remembers’ the history of the binary’s contraction. We illustrate the effect using a reduced model problem of an axisymmetric thin disc subjected at its inner edge to the weakening torque of an inspiraling binary. The model problem yields a new class of self-similar disc solutions, which capture salient features of the multidimensional hydrodynamics simulations. We use these solutions to derive variable AGN disc emission signatures within years to decades of massive black hole binary mergers in AGNs. Spectral changes of Mrk 1018 might have been triggered by an inspiral-merger event.