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Description
Periodic collisions between a star on an inclined orbit around a supermassive black hole and its accretion disk offers a promising explanation for the recently discovered X-ray quasi-periodic eruptions (QPEs) from galactic nuclei. 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 Monte Carlo radiation transport simulations which 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. For collision speeds v ≳ 0.15c and disk surface densities $\Sigma \sim 10^3$ g cm$^{−2}$ characteristic of those encountered by stellar orbits consistent with QPE recurrence times, the predicted transient light curves exhibit peak luminosities $\gtrsim 10^{42}$ erg s$^{−1}$ and Comptonized quasi-thermal spectra which peak at energies $h\nu \sim 100$ eV, 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 ineffcient 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 the star to have a large radius $R_{\star} \gtrsim 10R_{\odot}$, which may point to inflation of the star’s atmosphere from repeated collisions.