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(abridged) When a sufficiently massive satellite (or secondary) black hole isembedded in a gas disk around a (primary) supermassive black hole, it can openan empty gap in the disk. A gap-opening secondary close to the primary willleave an imprint in the broad component of the FeKa emission line, which variesin a unique and predictable manner. If the gap persists into the innermostdisk, the effect consists of a pair of dips in the broad line which rippleblue-ward and red-ward from the line centroid energy respectively, as the gapmoves closer to the primary. This ripple effect could be unambiguouslydetectable and allow an electromagnetic monitoring of massive black holemergers as they occur. As the mass ratio of the secondary to primary black holeincreases to q>0.01, we expect the gap to widen, possibly clearing a centralcavity in the inner disk, which shows up in the broad FeKa line component. Ifthe secondary stalls at >100r_{g} in its in-migration, due to low co-rotatinggas mass, a detectable ripple effect occurs in the broad line component on thedisk viscous timescale as the inner disk drains and the outer disk is dammed.If the secondary maintains an accretion disk within a central cavity, due todam bursting or leakage, a periodic 'see-saw' oscillation effect is exhibitedin the observed line profile. Here we demonstrate the range of ripple effectsignatures potentially detectable with Astro-H and IXO/Athena, and oscillationeffects potentially detectable with XMM or LOFT for a wide variety of mergerand disk conditions. Observations of the ripple effect and periodicoscillations can be used to provide an early warning of gravitational radiationemission from the AGN.

Ripple effects & oscillations in the broad FeKa line as a probe of massive black hole mergers