Radiation Transport Simulations of Quasiperiodic Eruptions from Star–Disk Collisions

Abstract Periodic collisions between a star on an inclined orbit around a supermassive black hole and its accretion disk offer a promising explanation for X-ray “quasiperiodic eruptions” (QPEs). Each passage through the disk midplane shocks and compresses gas ahead of the star, which subsequently re-expands above the disk as a quasi-spherical cloud. We present spherically symmetric Monte Carlo radiation transport simulations that follow the production of photons behind the radiation-mediated shock, Comptonization by hot electrons, and the eventual escape of the radiation through the expanding debris. Such 1D calculations are approximately justified for thin disks (scale-height h ≲ few × R ⋆ ), through which the star of radius R ⋆ passes more quickly than the shocked gas can flow around the star. For collision speeds v coll ≳ 0.15 c and disk surface densities Σ ∼ 10 3 g cm −2 characteristic of those encountered by stellar orbits consistent with QPE recurrence times, the predicted transient light curves exhibit peak luminosities ≳10 42 erg s −1 and Comptonized quasi-thermal (Wien-like) spectra that peak at energies hν ∼ 100 eV, which is broadly consistent with QPE properties. For these conditions, gas and radiation are out of equilibrium, rendering the emission temperature harder than the blackbody value due to inefficient photon production behind the radiation-mediated shock. The predicted eruptions execute counterclockwise loops in hardness–luminosity space, qualitatively similar to QPE observations. Reproducing the observed eruption properties (duration, luminosity, temperature) requires a large radius R ⋆ ≳ 10 R ⊙ , which may point to inflation of the star’s atmosphere from repeated collisions.