Radiation Transport Simulations of Quasi-Periodic Eruptions from Star-Disk Collisions

Periodic collisions between a star on an inclined orbit around a supermassive black hole and its accretion disk offers a promising explanation for X-ray "quasi-periodic eruptions" (QPEs). Each passage through the disk 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 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. Such one-dimension calculations are approximately justified for thin disks, through which the star of radius $R_{\star}$ passes faster than the shocked gas can flow around the star. For collision speeds $v_{\rm coll} \gtrsim 0.15 c$ 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 (Wien-like) spectra which peak at energies $h\nu \sim 100$ eV, broadly consistent with QPE properties. For these conditions, gas and radiation are out of equilibrium and the emission temperature is harder than the blackbody value due to inefficient photon production behind the shock. Alternatively, for higher disk densities and/or lower shock velocities, QPE emission could instead represent the comparatively brief phase shortly after shock break-out, though in this case the bulk of the radiation is thermalized and occurs in the ultraviolet instead of the X-ray band. In either scenario, reproducing the observed eruption properties (duration, luminosity, temperature) requires a large radius $R_{\star} \gtrsim 10R_{\odot}$, which may point to inflation of the star's atmosphere from repeated collisions.