Repeating X-ray Transients from Nuclei of External Galaxies as Extreme Mass Ratio Inspirals

Extreme Mass Ratio Inspirals (EMRIs) are binaries containing a massive black hole (≳a few hundred 1000 solar masses, Msol) and an orbiting smaller object (0.1-100 Msol). They are expected to be one of the primary sources of gravitational waves with space-based gravitational wave detectors that will start operating in the next decade. The identification of electromagnetic counterparts of these gravitational wave emitters would transform our understanding of supermassive black hole growth, probe dark energy and put fundamental constraints on gravity. I will present the various flavors of repeating nuclear transients that we have identified using multiwavelength studies of several classes of cosmic transients including stellar tidal disruption events, quasi-periodic eruptions from nuclei of external galaxies, and outbursts from active galactic nuclei. Using general relativistic hydrodynamic simulations I will present a model consisting of a supermassive black hole (SMBH) primary with a secondary object in a highly-inclined orbit which can explain all the observed properties of repeating transients. The unique signature of this SMBH disk + object interaction model is the presence of outflows which are launched every time the secondary punches through the inner accretion disc during each orbital period and is revealed as regular increases in X-ray absorption along our line of sight. I will end by briefly discussing the future prospects of some of these systems for multi-messenger studies.