Ripple effects and oscillations in the broad Fe Kα line as a probe of massive black hole mergers

When a sufficiently massive satellite (or secondary) black hole is embedded in a gas disc around a (primary) supermassive black hole, it can open an empty gap in the disc. A gap-opening secondary close to the primary will leave an imprint in the broad component of the Fe Kα emission line, which varies in a unique and predictable manner. If the gap persists into the innermost disc, 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 disc, which shows up in the broad Fe Kα line component. If the secondary stalls at ≥ 102rg in its in-migration, due to low corotating gas mass, a detectable ripple effect occurs in the broad line component on the disc viscous time-scale as the inner disc drains and the outer disc is dammed. If the secondary maintains an accretion disc 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 disc conditions, including gap width (or cavity size), disc inclination angle and emissivity profile, damming of the accretion flow by the secondary, and a minidisc 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 Fe Kα ripple effect or periodic oscillations will help in localizing their origin.