QPEs from EMRI Debris Streams Impacting Accretion Disks in Galactic Nuclei

Quasi-periodic eruption (QPE) sources in galactic nuclei are often associated with a stellar object orbiting a supermassive black hole with an hours to days period, which is brought in as an extreme mass-ratio inspiral (EMRI). In the presence of an accretion disk, repeated star-disk collisions lead to ablation of a small fraction of the stellar mass during each disk passage. We analytically track stellar debris as it is tidally stretched outside the EMRI’s Hill sphere, forming an elongated, dilute stream that collides with the disk half an orbit after the last star-disk encounter. For orbital periods ≳12 hr, the dilute stream is deflected at the disk surface by a strong shock, rather than penetrating it. Due to their low optical depth and prolonged interaction time, radiation from the shocked streams typically dominates over emission from shocked disk gas directly impacted by the star. We find that (1) QPE flare durations reflect the stream-disk collision timescale; (2) flare luminosities of 10⁴²⁻⁴³ erg s⁻¹, consistent with observed QPEs, are robustly produced; (3) soft X-ray flares with temperatures of ∼100 eV arise when the stream mass is sufficient to sustain a radiation-mediated shock at the collision interface. Higher-mass streams yield softer flares, typically outshone by the disk, while lower-mass streams result in collisionless shocks, which likely produce fainter and harder flares. We discuss observational implications of the temporal evolution of the underlying disk, assuming it is the remnant of a prior tidal disruption event in the same galaxy.