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

TL;DR

This study uses 3D radiation-magnetohydrodynamic simulations to explore how magnetic fields influence the formation and early evolution of accretion flows following tidal disruption events, revealing complex dynamics and outflow mechanisms.

Contribution

It provides new insights into the role of magnetic fields in TDE accretion flow formation, showing limited impact of MRI and magnetocentrifugal outflows, and detailing shock-driven outflows and angular momentum redistribution.

Findings

Magnetic fields increase debris stream thickness.

Radiation-driven outflows dominate early after collisions.

Accretion flow remains mostly eccentric despite magnetic stresses.

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.