Diverse Emission Patterns from Precessing Super-Eddington Disks Formed in Tidal Disruption Events

TL;DR

The paper addresses how misaligned, precessing super-Eddington accretion disks in TDEs imprint diverse multiwavelength variability patterns. By combining viewing-angle dependencies from aligned-disk radiative-transfer results with a rigid-body precession model, the authors categorize four variability classes (Smooth-TDEs, Dimmer, Blinker, Siren) and demonstrate an inverse X-ray/optical relation driven by disk geometry. They connect these patterns to observed events such as J0456-20, discuss rapid versus slow precession timescales via Lense-Thirring torques, and consider jet-disk interplay and alignment mechanisms, including potential observable signatures with future facilities like Einstein Probe and IXPE. The framework provides a practical approach to interpreting TDE light curves, constraining SMBH spin and disk tilt, and guiding targeted multiwavelength follow-ups.

Abstract

A tidal disruption event (TDE) occurs when a star passes within the tidal radius of a supermassive black hole (SMBH). In TDEs it is expected that the orbital angular momentum of the disrupted star is generally misaligned with the SMBH spin axis, which should result in a misaligned super-Eddington disk precessing around the SMBH spin axis due to the Lense-Thirring effect. In this paper, we investigate the distinct observational signatures produced from such TDE disks, by performing radiative transfer calculations upon previous super-Eddington disk simulations. We demonstrate that the precession of the disk and wind drive time-dependent obscuration and reprocessing of X-ray radiation. Depending on the orientation of the viewing angle of the observer and the tilt angle of the disk, four main types of variability are induced: 1) The smooth-TDEs: The emissions from these TDEs show no fluctuations; 2) The dimmer: The main emission type (X-ray or optical) stays the same, with small to moderate modulations of brightness; 3) The blinker: X-ray and optical emissions take turns to dominate in one cycle of precession, with dramatic changes in the X-ray fluxes. 4) The siren: X-ray and optical emissions take over each other twice per cycle, possibly with two different peak X-ray fluxes within one cycle. In all three scenarios, we observe an inverse correlation between X-ray and optical emissions. Our model provides a unified physical framework for interpreting TDE multi-wavelength variability through disk precession dynamics and gives an alternative interpretation to the interesting case of J045650.3-203750 which was suggested to be a repeated partial TDE previously.

Figures (9)

• Figure 1: Left: Schematic of a super-Eddington accretion disk with outflowing material (blue shaded region). When the observer's line of sight aligns with the outflow-cleared funnel, bright X-ray emission is detected. Conversely, observations along the equatorial plane are dominated by optical emission due to reprocessing. Right: $L_{\rm X, 0.2 - 10\ {\rm KeV}}$ and optical luminosity $L_{\rm o, bb}$ versus viewing angle $i$.
• Figure 2: Left: Sketch of disk precession geometry. As the disk precesses around the BH's spin vector ($\vec{J}_{\rm BH}$), the viewing angle $i(t)$ between the observer's line of sight ($\vec{r}_{\rm obs}$) and the disk orientation ($\vec{J}_{\rm disk}$) varies. The angle $\theta$ denotes the inclination between $\vec{J}_{\rm BH}$ and $\vec{J}_{\rm disk}$, while $\varphi$ represents the angle between $\vec{J}_{\rm BH}$ and $\vec{r}_{\rm obs}$. Both $\theta$ and $\varphi$ remain nearly constant during the procession cycle. Right: Evolution of viewing angle $i(t)$ of a precessing disk over one precession period. Upper, middle and lower panels illustrate the small, medium and large $\theta$ cases, respectively. The dashed gray line indicates the $90^{^\circ}$ viewing angle, beyond which the opposite side of the disk becomes visible as the disk viewing angle crosses this value. The blue line in the lower panel corresponds to the most extreme case with $\theta = \varphi = 90^{^\circ}$.
• Figure 3: Upper, middle and lower panels depict three distinct cases corresponding to different tilted angle $\theta$. The left and right panels illustrate the disk's positions at the two antipodal points of the precession cycle. The direction of precession is indicated by a circular arrow. The colored and labeled arrows elucidate the radiation patterns detectable by an observer as they scan different directions within a precession cycle.
• Figure 4: Schematic of observed precession patterns in an super-Eddington accretion disk. As the viewing angle varies across different ranges, the observer detects distinct signatures of disk precession, as indicated by the double arrows. A summary of these patterns is provided in Table \ref{['tab:patterns']}.
• Figure 5: Characteristic examples of the evolutions of X-ray luminosity $L_{\rm X, 0.2 - 2\ {\rm KeV}}$ (black lines) and optical luminosity $L_{\rm o, bb}$ (blue lines) due to disk precession over one precession period. Each panel shows the results corresponding to distinct viewing angle ranges. The left and right panels present the no/weak and large/extreme large modulation patterns, respectively. A narrow range of viewing angles arises when observing a precessing disk due to Small $\theta$ or small $\varphi$, resulting in the no/weak modulation. Otherwise, the wide range of viewing angles leads to large/extreme large amplitude of modulation.