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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. 

Abstract A subset of galaxies have dense nuclei, and when these galaxies are accreted and tidally stripped, the nuclei can masquerade as globular clusters in the halos of large galaxies. If these nuclei contain massive central black holes, some may accrete gas and become observable as active galactic nuclei. Previous studies have found that candidate stripped nuclei rarely host luminous X-ray sources, but these studies were typically restricted to both the most massive candidate nuclei and the most luminous X-ray sources. Here we use new and archival Chandra and XMM-Newton data to search for X-ray emission in a near-complete sample of massive globular clusters and candidate stripped nuclei in the nearest accessible elliptical galaxy, NGC 5128. This sample has the unique advantage that the candidate stripped nuclei are identified dynamically via elevated mass-to-light ratios (M/L_V). Our central result is that 5/22 ( $23_{-6}^{+11}$%) of the candidate stripped nuclei have X-ray sources down to a typical limit ofL_X∼ 5 × 10³⁶erg s⁻¹, a fraction lower than or comparable to that among massive clusters with normalM/L_V(16/41; $39_{-7}^{+8}$%). Hence we confirm and extend the result that nearly all X-ray sources in stripped nuclei are likely to be X-ray binaries rather than active galactic nuclei. If the candidate stripped nuclei have black holes of typical masses ∼2 × 10⁵M_⊙needed to explain their elevatedM/L_V, then they have typical Eddington ratios of ≲ 2 × 10⁻⁶. This suggests that it will be challenging to conduct an accretion census of wandering black holes around even nearby galaxies.