When a sufficiently massive satellite ( or secondary ) black hole is embedded in a gas disk around a ( primary ) supermassive black hole , it can open an empty gap in the disk .
A gap-opening secondary close to the primary will leave an imprint in the broad component of the Fe K \alpha emission line , which varies in a unique and predictable manner .
If the gap persists into the innermost disk , the effect consists of a pair of dips in the broad line which ripple blue-ward and red-ward from the line centroid energy respectively , as the gap moves closer to the primary .
This ripple effect could be unambiguously detectable and allow an electromagnetic monitoring of massive black hole mergers as they occur .
As the mass ratio of the secondary to primary black hole increases to q \gtrsim 0.01 , we expect the gap to widen , possibly clearing a central cavity in the inner disk , which shows up in the broad Fe K \alpha line component .
If the secondary stalls at \geq 10 ^ { 2 } r _ { g } in its in-migration , due to low co-rotating gas mass , a detectable ripple effect occurs in the broad line component on the disk 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 to dam bursting or leakage , a periodic ’ see-saw ’ oscillation effect is exhibited in the observed line profile .
Here we demonstrate the range of ripple effect signatures potentially detectable with Astro-H and IXO/Athena , and oscillation effects potentially detectable with XMM-Newton or LOFT for a wide variety of merger and disk conditions , including gap width ( or cavity size ) , disk inclination angle and emissivity profile , damming of the accretion flow by the secondary , and a mini-disk around the satellite black hole .
A systematic study of ripple effects would require a telescope effective area substantially larger than that planned for IXO/Athena .
Future mission planning should take this into account .
Observations of the ripple effect and periodic oscillations can be used to provide an early warning of gravitational radiation emission from the AGN .
Once gravitational waves consistent with massive black hole mergers are detected , an archival search for the FeK \alpha ripple effect or periodic oscillations will help in localizing their origin .