Radiation-magnetohydrodynamic Simulations of Accretion Flow Formation After a Tidal Disruption Event

TL;DR

This study uses 3D RMHD simulations with Athena++ to examine how strong magnetic fields influence the formation of the accretion flow after a tidal disruption event. By comparing toroidal, poloidal, and nonmagnetized debris streams, the authors find that magnetic pressure thickens the stream, reducing early radiative acceleration and outflows, but the subsequent evolution—dominated by stream-disk collisions and radiation pressure—produces a similar, eccentric disk and luminosity as in the nonmagnetized case. Reynolds stresses, not MRI-driven turbulence, primarily govern angular-momentum transport during the first week, while magnetically driven winds are not evident; luminosities reach near-Eddington values, with $L\sim4-6\times10^{44}$ erg s$^{-1}$ and $L/L_{Edd}\sim1-1.3$. The results emphasize collision-driven dissipation as the main energy source for TDE luminosities and reveal a strong azimuthal/polar dependence in the outflow and density structure, with implications for viewing-angle effects and multiwavelength signatures.

Abstract

We perform 3D radiation-magnetohydrodynamic simulations of the evolution of the fallback debris after a tidal disruption event. We focus on studying the effects of magnetic fields on the formation and early evolution of the accretion flow. We find that large magnetic fields can increase the debris stream thickness, moderately reducing the efficiency of the radiative acceleration of outflows during the first self-intersecting collisions. As gas accumulates and the collisions happen instead between the infalling stream and the accretion flow, magnetized and nonmagnetized systems evolve similarly at these early times: radiation-driven outflows dominate early after the initial stream-stream collision and a few days later, the accretion rate exceeds the mass outflow rate. We find that the MRI does not play a significant role in angular momentum transport and dissipation. Nor do we find evidence of a magnetocentrifugal driven outflow. Instead, collisions continue to dissipate kinetic energy into radiation that launches outflows and powers TDE luminosities reaching $L\sim4-6\times10^{44}$ erg s$^{-1}$. Shock-driven outflows and inflows redistribute angular momentum throughout the extent ($\sim50 r_s$) of the forming eccentric disk. Even in the presence of magnetic stresses, the accretion flow remains mostly eccentric with $e\sim0.2-0.3$ for $r\lesssim8r_s$ and $e\sim0.4-0.5$ for $10\lesssim r\,(r_s)\lesssim50$. Lastly, we find a polar angle-dependent density structure compatible with the viewing-angle effect, along with an additional azimuthal angle dependence established by the collisions.

Figures (16)

• Figure 1: From left to right: Density maps of HD, MHD-T, and MHD-P at the end of the simulations, $t=7$ days. The top row shows the midplane view, and the bottom row shows an azimuthal slice at the collision angle. The location of the azimuthal slice is marked as a grey dashed line on the midplane view panels. The original ballistic orbit is overplotted in green in the top MHD-T panel.
• Figure 2: Magnetic field strength at the end of the runs for MHD-T (left) and MHD-P (right). The streamlines show the projected magnetic field lines. The top row shows the midplane view, and the bottom row shows an azimuthal slice at the collision angle.
• Figure 3: Mass accretion rate measured at $r=3r_s$ as a function of time since the first stream self-intersection.
• Figure 4: Reynolds (left) and Maxwell (right) stresses averaged within $10$ degrees from the midplane, over azimuth and over the last two days of the simulation, normalized by average of the sum of gas and radiation pressures.
• Figure 5: Total accreted energy per unit time (black) and its components: kinetic energy (green), radiative energy (orange), magnetic energy (pink), and internal energy (red) for MHD-T, HD, and MHD-P, from left to right.