Theoretical Physics Colloquium: Stellar Destruction near Supermassive Black Holes

Centers of galaxies are fertile environments for a variety of dynamical processes, owing to the high density of stars and the presence of a central Supermassive Black Hole (SMBH). Fluctuations in the background potential resulting from two-body scatterings, as well as the dissipative nature of orbits in General Relativity through the emission of Gravitational Waves (GWs), shape the (quasi) steady-state distribution of orbits these systems settle to. I will introduce two main channels of stellar destruction caused by interaction with the central SMBH: Stars may be deflected onto a nearly radial trajectory where they are ripped apart by the SMBH’s tidal field, producing a luminous flare powered by the rapid fallback of stellar debris (known as Tidal Disruption Event, TDE), or alternatively, they may inspiral towards the SMBH on a nearly circular orbit until they begin to gradually shed their mass over many orbits (Extreme Mass Ratio Inspiral, EMRI). I will discuss the possible connection between these phenomena and a newly discovered class of repeating X-ray flares occurring in galactic nuclei, known as Quasi-Periodic Eruptions (QPEs). I will present a theoretical model for explaining the observations, involving a stellar EMRI interacting with the accretion flow produced by a recent TDE in the same galactic center. Finally, I will conclude by discussing the potential of QPEs and other repeating transients in galactic nuclei in testing our theoretical understanding of SMBHs and their environments - including measurements of their masses and spins, probing the nature of the accretion flow that forms around them, and constraining the dynamical processes that govern their engulfing stellar clusters.