Ripple effects & oscillations in the broad FeKa line as a probe of
  massive black hole mergers
  (abridged) 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 FeKa 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>0.01, we expect the gap to widen, possibly clearing a central
cavity in the inner disk, which shows up in the broad FeKa line component. If
the secondary stalls at >100r_{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 or LOFT for a wide variety of merger
and disk conditions. Observations of the ripple effect and periodic
oscillations can be used to provide an early warning of gravitational radiation
emission from the AGN.
