Tidal disruption events in active galactic nuclei: on orbital inclination and Schwarzschild apsidal precession

TL;DR

We address tidal disruption events in active galactic nuclei where stellar debris interacts with a pre-existing disk, and we systematically examine how the star’s orbital inclination and Schwarzschild apsidal precession influence debris dynamics and radiative signatures. Using meshless hydrodynamics with a generalized GR potential and an approximate radiative cooling term in GIZMO, we simulate an 8 Msun star disrupted near a 10^6 Msun black hole across inclinations from prograde to retrograde. The results reveal distinct inner-disk morphologies (central cavity with spirals for prograde vs. a tilted, debris-dominated inner disk for high inclinations), GR-driven alterations to debris trajectories and energy dissipation, and a robust two-phase light curve consisting of a precursor and a major fallback flare; UV/optical dips and quasi-periodic signals emerge depending on geometry. By synthesizing multi-band light curves and applying to AT2021aeuk, the work provides predictive diagnostics for AGN TDEs and links observed diversity to inclination and relativistic precession, with implications for the X-ray corona and QPO/QPE phenomenology in AGNs.

Abstract

Tidal disruption events (TDEs) in active galactic nuclei (AGNs) mark a regime where traditional vacuum models fail to capture the full dynamics, especially due to interaction between stellar debris and pre-existing accretion disks. We perform meshless hydrodynamic simulations incorporating both general relativistic (GR) effects and radiative cooling to study TDEs in AGNs with different orbital inclinations ($θ_{\rm inc}$) of the disrupted star, ranging from projected prograde to retrograde orbits. We post-process the simulations to derive multi-wavelength light curves and identify several distinct features in the light curves, including a precursor flare from early debris-disk collision and a major flare driven by fallback. The dynamics of the stellar debris and accretion disk, and subsequently the light curve features, are strongly affected by $θ_{\rm inc}$ and GR effects. Retrograde orbits ($θ_{\rm inc}=135^\circ$) yield a more luminous, shorter major flare and a more prominent precursor than prograde ones ($θ_{\rm inc}=22.5^\circ$). During fallback, prograde cases ($θ_{\rm inc} = 22.5^\circ$, $45^\circ$) develop a central cavity with spirals in the inner region of the AGN disk, leading to transient UV/X-ray suppression accompanied by oscillations, while higher inclinations ($θ_{\rm inc}=90^\circ$, $135^\circ$) form a gradually tilting inner disk, potentially causing UV/X-ray dips via geometric effects at certain viewing angles. Relativistic apsidal precession alters stream collisions, producing structural differences in the inner disk, outer disk, and debris compared to Newtonian cases, and drives quasi-periodic signals in prograde configurations. These results provide predictive diagnostics for identifying AGN TDEs and interpreting observed light-curve diversity.

Figures (16)

• Figure 1: Radial profiles of (a) density and (b) temperature of the AGN disk during relaxation, with colors indicating different times. Panel (c) shows the time evolution of the disk's total energy. Gray circles represent simulation data, while the black solid line indicates the expected quasi-steady energy loss rate, estimated from the viscous heating rate $Q_{\rm visc}$ (Equation \ref{['viscous_heating']}). The red vertical line marks $t\approx80$ days, when viscous heating becomes nearly balanced by radiative cooling. The red arrow in panel (c) indicates $t\approx90$ days, at which the disk is considered to be relaxed and the corresponding profiles are adopted as the initial condition.
• Figure 2: A sketch map illustrating the inclined stellar orbits adopted in this work. Each orbit is color-coded according to its inclination angle, with an arrow indicating the orientation of motion. The direction of rotation of the accretion disk is marked with a black arrow for reference.
• Figure 3: Time evolution of the log-scaled column density in an arbitrary inclined view of our fiducial model R90 ($\theta_{\rm inc}=90^\circ$) over $\sim$100 days. Each snapshot presents a zoom-in view of the inner region on the left and a full view of the co-evolution between the outer disk and the stellar debris on the right. Time since the start of the simulation is labeled in each snapshot. Spatial scale bars for the zoomed-in and full views are displayed beneath the figure. (a)-(b): the star is disrupted and tears through the disk, leaving a gap on the outer disk along its trajectory. (c): The debris separates from the disk, with its leading end beginning to fall back toward the pericenter. This marks the end of continuous debris-disk interaction and the onset of the fallback phase. (d)-(e): The fallback stream rapidly reaches its peak intensity and subsequently weakens as the debris continues to be stretched by tidal forces. An inclined inner disk forms during the fallback phase. An animated version of this figure is available https://www.youtube.com/watch?v=hIt90J9X8uY, showing the face-on column density from 0 to $\sim100$ days across four inclination angles ($\theta_{\rm inc}=22.5^\circ,\ 45^\circ,\ 90^\circ$, and $135^\circ$).
• Figure 4: Face-on views of the evolution of the log-scaled column density of the inner region ($R < 10^3 R_g$) of the disk for models with $\theta_{\rm inc} = 22.5^\circ$, $45^\circ$, $90^\circ$, and $135^\circ$. The $22.5^\circ$ and $45^\circ$ models develop a central cavity and prominent spiral arms, while the $90^\circ$ and $135^\circ$ models form a gradually tilting inner disk. The time since the start of the simulation for each snapshot is labeled on the top right of each row.
• Figure 5: Time evolution of the radial surface density profiles for models with stellar orbital inclinations of $22.5^\circ$, $45^\circ$, $90^\circ$, and $135^\circ$. The plot range is limited to the inner region ($R<10^3R_g$). The total surface density, AGN disk contribution, and stellar debris contribution are shown by the solid gray, solid black, and dash-dotted black lines, respectively. The pericenter distance as well as the impact radius ($R_p$) is indicated by dashed red vertical lines. The corresponding time for each panel is labeled in the upper-left corner.