Unlocking the Quasiperiodic Eruption Mystery: Star–Disk Collisions in Realistic Active Galactic Nucleus Disks

Quasiperiodic eruptions (QPEs) are luminous, recurring soft X-ray outbursts (∼100 eV) observed in the nuclei of low-mass galaxies. They display two remarkable trends: outburst durations are ∼10%–20% of the recurrence timescale, and longer bursts are more luminous. A promising theory that naturally explains the quasiperiodicity invokes collisions between a star on an extreme mass-ratio inspiral (EMRI) orbit and the accretion disk around the supermassive black hole. However, it remains unclear how this model reproduces the observed trends. We therefore carry out 2D, multifrequency radiation-hydrodynamic (RH) simulations of star–disk collisions. Crucially, we adopt a realistic circumnuclear disk structure from previous radiation MHD simulations of sub-Eddington accretion disks. We find that the thick, puffed-up disk atmosphere, extending to z / r ∼ 1, causes different portions of the bow shock to break out at different times, producing prolonged thermal emission as the shock emerges through the breakout surface at z / r ∼ 0.7. The X-ray flare duration is set by the shock propagation time through the optically thick disk—∼10% of the orbital timescale, reproducing the observed duty cycle. A more oblique star–disk interaction yields a longer, more luminous flare. The realistic disks also exhibit a surface density Σ ∝ r 2 , giving a collisional energy E ∝ P 2/3 that may explain the luminosity–period trend, especially for the weaker QPEs. Overall, we suggest that a realistic circumnuclear disk structure can explain several observed QPE trends—and that QPEs may, in turn, constrain the disk structure.