Repeating Extragalactic Nuclear Transients as Extreme Mass Ratio Binaries

Extreme Mass Ratio binaries are systems containing a massive black hole (>10,000 Msolar) and a closely orbiting smaller object (0.1-1000 Msolar). If the companion is also a black hole they can produce gravitational waves, potentially detectable with the space-based detectors that will start operating in the next decade. The identification of electromagnetic counterparts of such gravitational wave emitters will transform our understanding of supermassive black hole growth, probe dark energy, and put fundamental constraints on gravity. Alternatively, if the companion is a star, such systems allow us to study the orbital dynamics around massive black holes in unprecedented detail. I will present an overview of the various flavors of repeating nuclear transients that we have identified using multi-wavelength studies of several classes of astrophysical transients including stellar tidal distribution events, quasi-periodic eruptions, from nuclei of external galaxies, and outbursts from active galactic nuclei (AGN). I will also present state-of-the-art general relativistic hydrodynamic simulations of objects embedded in AGN disks and argue that in some cases these repeating transients could be double compact object binaries with direct implications for multi-messenger astronomy. I will end by highlighting the prospects they hold for the coming decade.