How Flat is a Plateau? Evolution of Late-Time TDE Disks

TL;DR

This study tests whether late-time plateaus in tidal disruption event disks are truly time-independent or exhibit evolution. By combining a theory-agnostic phenomenological light-curve analysis with a magnetically elevated, self-consistent $α$-disk model, the authors extract SMBH masses, disrupted-star masses, and the viscous stress parameter $α$ from a sizable TDE sample. They find that roughly one-third of events favor evolving plateaus, one-third are consistent with flat plateaus, and one-third show no statistically significant plateau, while a subset of plateaus can be well fit by the magnetized-disk framework, yielding $α$ values in the range $10^{-3}$ to $0.4$ and SMBH masses broadly compatible with scaling relations. The work also predicts faster disk precession in magnetized disks and estimates that late-time precession cycles are typically a few to ten, offering observable signatures to test angular-momentum transport and alignment physics in accretion disks.

Abstract

Late-time light curve plateaus in tidal disruption events (TDEs) are often approximated as flat and time-independent. This simplification is motivated by theoretical modeling of spreading late time TDE disks, which often predicts slow light curve evolution. However, if time evolution can be detected, late-time light curves will yield more information than has been previously accessible. In this work, we re-examine late-time TDE data to test how well the flat plateau assumption holds. We use Markov Chain Monte Carlo to estimate the maximum likelihood for a family of theory-agnostic models and apply the Akaike information criterion to find that that roughly one third of our sample favors evolving plateaus, one third favors truly flat plateaus, and one third shows no statistically significant evidence for any plateau. Next, we refit the TDEs that exhibit statistically significant plateaus using a magnetically elevated $α$-disk model, motivated by the lack of clear thermal instability in late time TDE light curves. From these model-dependent fits, we obtain estimates for the supermassive black hole (SMBH) mass, the mass of the disrupted star, and the $α$ parameter itself. Fitted $α$ values range from $10^{-3}$ to 0.4 (the mean fitted $α=10^{-1.8}$, with scatter of 0.6 dex), broadly consistent with results from magnetohydrodynamic simulations. Finally, we estimate the timescales of disk precession in magnetically elevated TDE models. Theoretically, we find that disk precession times may be orders of magnitude shorter than in unmagnetized Shakura-Sunyaev disks, and grow in time as $T_{\rm prec}\propto t^{35/36}$; empirically, by using fitted $α$ parameters, we estimate that late time disks may experience $\sim$few-10 precession cycles.

Figures (8)

• Figure 1: Three examples of observed multi-band light curves with the plateau phenomenological model with the lowest AIC: no plateau (single power-law; top), a flat plateau (middle), and a decaying plateau (bottom). The fitted models for early-time decay are shown as dashed lines. For presentation purposes, the data are binned in intervals of 3 days for $t<100$ days, 10 days for $t<365$ days, and 30 days for $t>365$ days.
• Figure 2: Characteristic plateau luminosities plotted against SMBH masses estimated from galaxy scaling relations ($M_\bullet$-$\sigma$ scaling relation in circles and $M_\bullet$-$M_{\rm gal}$ in triangles). The color coding indicates the qualitative nature of the best-fit (i.e. AIC minimizing) model. The top panel shows the nature of the plateau, while the bottom panel shows the early time. TDEs best fitted without a plateau (a single power-law decay) are shown in black; true (time-independent) plateaus are shown in red; and slowly evolving plateaus are shown in green. Events whose best-fit models include a plateau with early-time exponential decay are shown in blue, while those with early-time power-law decay are shown in purple.
• Figure 3: Characteristic plateau luminosities plotted against MBH masses estimated from galaxy scaling relations as in \ref{['fig:flat vs evolved']}. Here all late-time plateau light curves have been fit with a power-law $L_{\rm late} \propto t^{-p_{\rm cuesta}}$ using \ref{['eq: late model PL']}, and the color coding represents the best-fit power law index $p_{\rm cuesta}$. The points in grey are those TDEs that show no statistical evidence for a plateau.
• Figure 4: SMBH mass $M_\bullet$ as a function of the disrupted star’s mass $m_\star$, fitted using the magnetically elevated disk model. The color of each point indicates the Shakura–Sunyaev $\alpha$ parameter. The gray line marks the Hills mass. TDEs that cannot be explained by our model and are therefore excluded from the subsequent figures are shown with gray outlines.
• Figure 5: SMBH mass fitted from the magnetized disk model as a function of the SMBH mass from scaling relations (the $M_\bullet$-$\sigma$ scaling relation is shown as circles and $M_\bullet$-$M_{\rm gal}$ as triangles). The gray dashed line indicates where the SMBH masses are equal between the two approaches. The gray points are those TDEs that are excluded because our disk model is not a correct description of the late-time observations, hence their $M_\bullet$ values (inferred from disk fitting) should not be trusted.