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

We perform 3D radiation-magnetohydrodynamic simulations of the evolution of the fallback debris after a tidal disruption event (TDE). 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 magnetorotational instability 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 ∼ 4–6 × 10⁴⁴ erg s⁻¹. Shock-driven outflows and inflows redistribute angular momentum throughout the extent (∼50r_s) of the forming eccentric disk. Even in the presence of magnetic stresses, the accretion flow remains mostly eccentric with e ∼ 0.2–0.3 for r ≲ 8r_s and e ∼ 0.4–0.5 for 10 ≲ r (r_s) ≲ 50. 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.

We thank the anonymous referee for providing helpful comments that improved this work. We thank Kengo Tomida for his helpful collaboration in solving initial problems with the simulation setup. We also thank Phil Chang, Edwin (Chi-Ho) Chan, Eric Coughlin, and Lizhong Zhang for useful discussions. We thank Zhaohuan Zhu for providing the opacity tables. X.H. is supported by the Sherman Fairchild Postdoctoral Fellowship at the California Institute of Technology. This work used Stampede 3 at Texas Advanced Computing Center through allocation TG-PHY240041 from the Advanced Cyber infrastructure Coordination Ecosystem: Services & Support (ACCESS) program. This work also made use of UVA’s High-Performance Computing systems Rivanna and Afton. Support for this work was provided by the National Science Foundation under grant 2307886. X.H. appreciates the hospitality and interactions during the tde24 workshop, which is supported by the NSF grant PHY-2309135 to the Kavli Institute for Theoretical Physics (KITP). The Center for Computational Astrophysics at the Flatiron Institute is supported by the Simons Foundation. We acknowledge funding from the Virginia Institute for Theoretical Astrophysics (VITA), supported by the College and Graduate School of Arts and Sciences at the University of Virginia.