Speaker
Description
A small fraction of the stars in galactic nuclei migrate towards the central supermassive black hole (SMBH) via gravitational wave emission, arriving to small scales on low-eccentricity orbits ("extreme mass-ratio inspirals"; EMRIs). If a gaseous accretion disk suddenly forms around the SMBH during this gradual inspiral (e.g. by an independent tidal disruption event), then twice-per-orbit periodic collisions are expected to occur between the star and the disk. Each time the star passes through the disk, it shocks and ejects hot debris above and below the midplane, powering a luminous burst of hard radiation. I will argue that such star-disk collisions provide a natural explanation for the recently discovered phenomenon of X-ray "quasi-periodic eruptions" (QPE) in low-mass galactic nuclei. If this explanation is correct, QPEs offer a new probe of the EMRI populations in galactic nuclei; the structure of radiation-dominated accretion disks; and strong-gravity effects such as apsidal/nodal orbital precession.
