Prolonged fallback and late-time rebrightening episodes in stellar tidal disruptions as imprints of a galactic environment

TL;DR

This work extends classical TDE theory by incorporating the host galaxy's extended gravitational potential into the debris fallback dynamics, using a double power-law density to model $\Phi_{gal}$ and deriving a closed-form potential via incomplete Beta functions. The authors demonstrate that environmental gravity can induce light-curve features beyond the standard $\dot{M}\propto t^{-5/3}$ decay, including late-time shallow slopes and rebrightenings, with the most pronounced effects for lower-mass black holes and denser environments. Through a four-parameter parameter study ($m_{bh}, m_{gal}, b, \delta$) across multiple galactic profiles, they map regimes producing non-Keplerian signatures and test the framework on the TDE J1331, finding that reproducing its ~30-year rebrightening would require unrealistically compact hosts, limiting the model's applicability for early-time events. The results imply that TDE light curves can encode information about galactic structure, providing a potential avenue to probe black hole demographics and the architecture of galactic environments with future long-baseline observations.

Abstract

We extend the classical Keplerian framework of existing analytic TDE models by incorporating the gravitational potential of a spherically symmetric galactic mass distribution. We then demonstrate that this broader structure imprints light curve features beyond the predictive scope of traditional models, such as phases of shallower-than-standard decay and late-time rebrightening episodes. Importantly, our framework predicts the occurrence of environment-induced rebrightenings but only on very long timescales, unless the host environment is unrealistically ultra-compact. This means the early evolution of TDEs occurring in typical galaxies is essentially untouched by the host potential, which explains why Keplerian models have been so successful in describing the first few years after disruption. To illustrate, we applied our model to the TDE candidate eRASSt J133157.9-324321 (J1331), the event with the longest reported rebrightening interval, and find that even matching its ${\sim}$30-year rebrightening would demand an implausibly dense host. This demonstrates the limits of environmental effects as an explanation for early rebrightenings reported in the literature. More broadly, our work shows that while the host galaxy leaves TDEs nearly Keplerian at early times, it actively shapes their long-term evolution and can drive departures from the canonical $t^{-5/3}$ decay law. These delayed signals give us a testable way to see how the host galaxy shapes the event, and they may even offer clues about the galaxy's underlying structure.

Figures (8)

• Figure 1: Upper panel: Time evolution of the mass accretion rate for two different IMBH-Hernquist systems. The black hole mass is set to $M_{\mathrm{bh}} = 10^4 \, M_{\odot}$ and the scale length of the Hernquist environment is set to $b \approx 10^7 R_{\odot}$. The total galactic mass $M_{\mathrm{gal}}$ of the hosts are indicated in the plot legends. Lower panel: Corresponding time evolution of the power-law index $\mu$ describing the decay slope. The region of shallower decay is characterized by a less negative slope ($-5/3 < \mu < 0$) whereas the rebrightening episode corresponds to a positive slope ($\mu > 0$). For reference, the dotted lines in the background shows the canonical $t^{-5/3}$ decay slope expected in standard TDEs.
• Figure 2: Schematic diagram of an extended black hole system. A central black hole is surrounded by a spherically symmetric distribution of mass, modeled as concentric shells with increasing density toward the center. The tidal radius is shown in red, and the black dashed curve traces the path of an incoming star.
• Figure 3: Left: Time evolution of the mass accretion rate for a range of black hole masses. The black hole is embedded in a Hernquist environment with a fixed scale radius of $10^8 R_{\odot}$, and the penetration factor is set to $\delta = 1$. Dashed lines indicate the Eddington limit corresponding to each black hole mass, while the dotted background curve shows the canonical fallback rate scaling of $t^{-5/3}$ for reference. Right: Corresponding evolution of the instantaneous power-law index. The dotted line marks the standard $-5/3$ slope, and the solid horizontal line indicates a slope of $0$, distinguishing rebrightening phases from shallow decay intervals.
• Figure 4: Left: Time evolution of the mass accretion rate across varying galactic masses. The Keplerian light curve (black dashed curve) is also plotted for reference. The black hole mass is fixed at $10^4 M_{\odot}$, the penetration factor is set to $1$, and the scale length of the Hernquist environment is set to $10^8 R_{\odot}$. The dotted line in the background provides a comparison to the canonical scaling of $t^{-5/3}$. Right: Corresponding evolution of the instantaneous power-law index. The dotted line marks the $-5/3$ slope and the solid line represents a slope of $0$.
• Figure 5: Same as Figure \ref{['fig:galmass']}, but now varying the scale radius $s$ while keeping the galactic mass fixed at $m_{\mathrm{gal}} = 10^7$.