Andrew Mummery, Eliot Quataert
6 min
Tidal disruption events (TDEs) occur when a star is shredded by a supermassive black hole, forming a misaligned accretion disk. Because the star's original orbital plane is random relative to the black hole's spin axis, Lense-Thirring torques should theoretically force the disk to precess. This precession would cause the inner disk to rock back and forth, creating a periodic modulation in X-ray emission. However, despite observing hundreds of TDEs, clear, long-lived periodic signals remain elusive. This paper argues that this absence is not a mystery, but a natural consequence of the physics of thick, super-Eddington accretion disks.
The authors demonstrate that for a disk to precess as a solid body, it must be thick (H/R ~ 1). In this state, the disk is subject to rapid accretion. To conserve angular momentum as material flows inward toward the black hole, the disk must redistribute angular momentum by pushing other material outward. This causes the disk to spread rapidly. Because the precession frequency is highly sensitive to the distribution of angular momentum—specifically, the ratio of material near the inner edge to the total disk mass—this rapid spreading causes the precession period to increase aggressively over time. This phenomenon, known as chirping, means the precession period changes so quickly that the disk cannot complete more than a few cycles before the signal becomes unobservable.
A similar mechanism prevents TDE disks from aligning with the black hole's spin axis during the super-Eddington phase. Alignment requires the disk to dissipate the energy stored in its internal twists, a process that takes time. Because the alignment timescale is also strongly chirped by the same disk-spreading mechanism, the disk effectively never has enough time to align. Consequently, TDE disks are predicted to enter their subsequent thin-disk phase with a global, quasi-steady warp profile. This suggests that while periodic precession signals are unlikely to be found, the imprints of relativistic Lense-Thirring physics may instead be hidden in the warped geometry of late-stage TDE disks.
Tidal disruption event X-ray light curves do not appear to show clear signs of periodic modulation. This is naively somewhat surprising since the stars that are disrupted originate at large scales in the galaxy where they cannot know about the orientation of the black hole spin axis. This should lead to the formation of a misaligned disk that precesses due to Lense-Thirring torques, modulating X-ray emission from the inner disk regions. We argue that in fact the properties which are required for solid-body precession, namely a thick $(H/R\sim {\mathcal O}(1))$ disk, naturally lead to rapid precession period change, with a per-period increase of $ΔT_{\rm prec}/T_{\rm prec} \sim {\mathcal O}(1-10)$, as the disk spreads to larger radii to conserve angular momentum. In other words the precession of thick TDE disks is aggressively chirped, washing out any possibility of observing multiple cycles other than for fine tuned regions of parameter space. The global disk alignment timescale is equally strongly chirped by the exact same mechanism, meaning that TDE disks will not in general align with the black hole spin axis during a super-Eddington phase, and should generically show global quasi-steady warp profiles at the beginning of any thin disk phase. These results have important implications for interpreting timing features associated with TDE disks, including models for quasi-periodic X-ray eruption timing phenomenology, and the observational properties of TDE disks in the initial transition to the thin disk phase.
Alex: [pointing to the paper] It’s a compelling case. If you want to see how they model the specific drift rates, check out the derivation in section three of the paper.
Sam: [short sign-off] I’ll pull it up now.
Alex: [measured, grounding the finding] The precession frequency of a tidal disruption event disk is aggressively chirped because the disk must expand to conserve angular momentum, causing the precession period to diverge rapidly. That is the central argument from Mummery and Quataert’s recent work.
Sam: [curious, probing the mechanism] So the signal isn't just buried in noise—it’s being physically erased? If the disk needs to be thick to precess, but that thickness forces it to spread out and dump its angular momentum, that kills the precession?
Alex: [nodding, confirming the logic] Exactly. The precession frequency depends on the angular momentum at the inner edge. As the disk spreads, that fraction drops, and the precession period diverges.
Sam: [analytical, testing the implication] It’s like a figure skater forced to extend their arms wide mid-spin; the rotation slows down instantly. Does this mean the lack of periodic X-ray modulation in TDEs is actually a confirmation of this disk evolution?
Alex: [deliberate, clarifying] It suggests the missing periodicity is a feature. The conditions required for coherent Lense-Thirring precession—a thick, super-Eddington disk—are the exact conditions that force the disk to expand and destroy that coherence.
Sam: [thoughtful, connecting the dots] And the authors argue this applies to the alignment timescale too? So, if a TDE disk survives its initial phase, it should enter the thin-disk phase already warped and misaligned with the black hole spin.
Alex: [measured, expanding] Correct. The paper predicts TDE disks will generically show global, quasi-steady warp profiles at the start of any thin-disk phase, because alignment is stalled by that same angular momentum redistribution.
Sam: [probing, checking for robustness] Where would a referee push back? If cooling is inefficient or wind dynamics are complex, could that save the precession signal?
Alex: [measured, acknowledging the limitation] The authors acknowledge that. They treat their super-Eddington model as a best-case scenario. Even then, the period drift is so extreme—on the order of ten times the initial period—that a coherent signal remains statistically unlikely.
Sam: [reflective, summarizing] So, we shouldn't expect stable periodicities in early-time TDE X-ray data. The physics of the disk's own growth is fundamentally hostile to the clock-like signals we’ve been looking for.
Alex: [pointing to the paper] It’s a compelling case. If you want to see how they model the specific drift rates, check out the derivation in section three of the paper.
Sam: [short sign-off] I’ll pull it up now.
Alex: [measured, grounding the finding] The precession frequency of a tidal disruption event disk is aggressively chirped because the disk must expand to conserve angular momentum, causing the precession period to diverge rapidly. That is the central argument from Mummery and Quataert’s recent work.
Sam: [curious, probing the mechanism] So the signal isn't just buried in noise—it’s being physically erased? If the disk needs to be thick to precess, but that thickness forces it to spread out, that kills the precession?
Alex: [nodding, confirming the logic] Exactly. The precession frequency depends on the angular momentum at the inner edge. As the disk spreads, that fraction drops, and the precession period diverges.
Sam: [analytical, testing the implication] It’s like a figure skater forced to extend their arms wide mid-spin; the rotation slows down instantly. Does this mean the lack of periodic X-ray modulation in TDEs is actually a confirmation of this disk evolution?
Alex: [deliberate, clarifying] It suggests the missing periodicity is a feature. The conditions required for coherent Lense-Thirring precession—a thick, super-Eddington disk—are the exact conditions that force the disk to expand and destroy that coherence.
Sam: [thoughtful, connecting the dots] And the authors argue this applies to the alignment timescale too? So, if a TDE disk survives its initial phase, it should enter the thin-disk phase already warped and misaligned with the black hole spin.
Alex: [measured, expanding] Correct. The paper predicts TDE disks will generically show global, quasi-steady warp profiles at the start of any thin-disk phase, because alignment is stalled by that same angular momentum redistribution.
Sam: [probing, checking for robustness] Where would a referee push back? If cooling is inefficient, could that save the precession signal?
Alex: [measured, acknowledging the limitation] The authors acknowledge that. They treat their super-Eddington model as a best-case scenario.