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

Understand

(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.