Radiation-hydrodynamics of star-disc collisions for quasi-periodic eruptions

TL;DR

This work tests the star–disc collisions scenario for quasi-periodic eruptions by performing the first 3D radiation-hydrodynamics simulations of a star crossing an accretion disc. A bow shock forms and heats gas beyond the direct collision column, driving two asymmetric outflows due to directional momentum injection and wake dynamics; the forward outflow is typically faster, more massive, and more luminous than the backward one, producing a natural mechanism for alternating strong–weak QPE flares. The simulated bolometric luminosities peak near $L \sim 10^{41}\,\mathrm{erg\,s^{-1}}$ with a forward/backward contrast of about 2, and the peak temperatures reach $k_B T_{\rm eff} \approx 29\,\mathrm{eV}$ for the forward flow, though LTE assumptions likely understate the spectral hardness compared with some QPE observations. Limitations include the rigid-star approximation and LTE radiative transfer, motivating future work with non-LTE physics and a broader parameter study to robustly connect with the observed diversity of QPEs.

Abstract

Quasi-periodic eruptions (QPEs) are recently discovered transients of unknown nature occurring near supermassive black holes, which feature bright X-ray bursts separated by hours to days. A promising model for QPEs is the star-disc collisions model, where a star repeatedly interacts with an accretion disc around a black hole, creating shocks that expel dense outflows of gas from which radiation emerges. We investigate the dynamics of the star-disc collisions, the properties of the outflows, and the resulting radiation signatures. Our study focuses on the generic case where the star remains unperturbed by the collision and the stellar crossing time through the disc is sufficiently long for shocked gas to flow around the star. We performed a three-dimensional (3D) radiation-hydrodynamics simulation of the star-disc collision. The star was modeled as a solid, spherical body, and the interaction was simulated for a small, local section of the accretion disc. We found that star-disc collisions generate a nearly paraboloidal bow shock. The heating of gas is not confined to the column of gas directly ahead of the star but also extends laterally as the shock front expands sideways while traveling with the star. As the star crosses the disc, it injects momentum preferentially along its direction of motion, leading to an asymmetric redistribution of energy and momentum. As a result, two outflows emerge on opposite sides of the disc with different properties: the forward outflow expands faster, contains more mass, carries more energy, and is about twice as luminous as the backward outflow. Our findings suggest that the asymmetry in outflow properties and luminosity arises naturally from the collision dynamics, offering a possible explanation for the alternating strong-weak flare patterns observed in several QPE sources.

Figures (13)

• Figure 1: Schematic illustration of the star-disc collisions model for QPEs. The red circle, orange ellipse, and black circle represent the star, the accretion disc, and SMBH, respectively. The star follows an orbit represented by the gray ellipses, intersecting the accretion disc twice per orbital period. Each collision generates a flare of electromagnetic (EM) radiation, resulting in a luminosity $L$ seen by a distant observer, as illustrated in the graph on the left-hand side. If one of the outflows is more luminous, the observer would see a recurrent pattern of brighter and dimmer flares. The inset on the right zooms into the collision region, marked by a dashed rectangle, highlighting the initial setup of our simulations: a localized section of the disc with horizontal and vertical extents of $L$ and $2H$, respectively, and a star, with radius $R_\star$, moving perpendiculary toward the disc with velocity $v_\star$.
• Figure 2: Gas density slices in the $yz$-plane at $x=0$ at different times. The white circle denotes the star.
• Figure 3: Radiation energy density slices in the $yz$-plane at $x=0$ at different times. The white circle denotes the star, while the dashed gray lines mark the outer edges of the accretion disc.
• Figure 4: Gas density (left panel) and radiation energy density (right panel) slices in the $yz$-plane at $x=0$ at $t/t_\rm{cr}=3$. The white circle denotes the star, while the dashed gray lines on the right panel mark the outer edges of the accretion disc. The pink and brown arrow lines in the left panel indicate the lateral shock front distance $r_\rm{sh}^\perp$.
• Figure 5: Shock heating rate $\dot{E}$ as a function of time. The solid blue line represents the total $\dot{E}$. Other lines correspond to $\dot{E}$ calculated for gas inside different regions relative to the vertical and lateral distance from the CoM of the star, located at $z_\star$ at time $t$: gas inside a cylindrical column directly ahead of the star with ${r}_\perp\leq R_\star$ and $z\leq z_\star$ (dash-dotted green), gas outside of the cylindrical column intercepted by the star with ${r}_\perp> R_\star$ (dotted red), and gas inside a cylindrical column directly behind the star with ${r}_\perp\leq R_\star$ and $z> z_\star$ (double-dotted dashed magenta). The coloured vertical lines correspond to specific stages of star-disc collisions (see Figures \ref{['fig:qpe_sim_density']} and \ref{['fig:qpe_sim_erad']}).