Speaker
Description
In recent years, a mysterious new class of astrophysical objects has been uncovered. These objects are spatially coincident with the nuclei of external galaxies and exhibit X-ray variations that repeat over timescales ranging from minutes to months. They manifest in three distinct ways in the data: stable quasi-periodic oscillations (QPOs), quasi-periodic eruptions (QPEs), and quasi-periodic outflows (QPOuts). QPOs are systems that display smooth, recurrent variations in X-ray brightness, while QPEs are characterized by sudden changes resembling eruptions. QPOuts represent systems that show repeating outflows moving at mildly relativistic speeds of approximately $0.1-0.3c$, where c is the speed of light. The underlying physical mechanisms driving these phenomena, collectively called as repeating nuclear transients (RNTs), are a topic of intense debate, with most models suggesting that they originate from either instabilities within the inner accretion flow or from orbiting objects. There is significant excitement surrounding the latter class of models, as it has been proposed that some repeating systems could host extreme mass-ratio inspirals, potentially detectable by upcoming space-based gravitational wave interferometers. This could pave the way for a new era of “persistent” multi-messenger astronomy. I will briefly summarize the most important observational facts about RNTs and discuss the most promising EMRI scenarios, also presenting supporting GRMHD simulations.
