Unlocking the QPE Mystery: Star-Disk Collisions in Realistic AGN Disks
Abstract
Quasi-periodic eruptions (QPEs) are luminous, recurring soft X-ray outbursts 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 quasi-periodicity 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 two-dimensional, multi-frequency radiation hydrodynamic (RH) simulations of star--disk collisions. Crucially, we adopt a more 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--a ∼10% of the orbital timescale, reproducing the observed duty cycle. A more oblique star-disk interaction yields a longer, more luminous flare. The realistic AGN disk models also exhibit a surface density Σ∝ r2, giving a collisional energy E∝ P2/3 that may explain the luminosity--period trend, especially for the weaker QPEs. Overall, we suggest that a more realistic circumnuclear disk structure can explain several observed QPE trends-and QPEs may, in turn, constrain the disk structure.
Versions
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BibTeX
@misc{zhu2026unlocking,
title = {{Unlocking the QPE Mystery: Star-Disk Collisions in Realistic AGN Disks}},
author = {Zhaohuan Zhu and Xiaoshan Huang and Yan-Fei Jiang and Shunquan Huang},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.03011},
eprint = {2609.03011},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.03011},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Unlocking the QPE Mystery: Star-Disk Collisions in Realistic AGN Disks
AU - Zhaohuan Zhu
AU - Xiaoshan Huang
AU - Yan-Fei Jiang
AU - Shunquan Huang
PY - 2026
DA - 2026/09/02
PB - arXiv
DO - 10.48550/arxiv.2609.03011
UR - https://arxiv.org/abs/2609.03011
AB - Quasi-periodic eruptions (QPEs) are luminous, recurring soft X-ray outbursts 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 quasi-periodicity 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 two-dimensional, multi-frequency radiation hydrodynamic (RH) simulations of star--disk collisions. Crucially, we adopt a more 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--a ∼10% of the orbital timescale, reproducing the observed duty cycle. A more oblique star-disk interaction yields a longer, more luminous flare. The realistic AGN disk models 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 more realistic circumnuclear disk structure can explain several observed QPE trends-and QPEs may, in turn, constrain the disk structure.
ER -