Hydrodynamic Simulations of Tidal Disruption Encores

TL;DR

This work investigates tidal disruption encore events (TDEEs) in nuclear star clusters by performing 3D hydrodynamic AREPO simulations of Sun-like stars disrupted by stellar-mass black holes within the potential of a more massive MBH. The study identifies two robust morphologies—Direct Encore, where debris plummets toward the MBH and shocks generate prompt emission, and Ring Encore, where debris circularizes into a ring and emits on longer viscous diffusion timescales—producing characteristic double-flare light curves with luminosities in the range $L\sim 10^{40}-10^{42}$ erg s$^{-1}$. Bolometric luminosities are estimated via post-processing radiation diffusion in optically thick debris, complemented by viscous timescale assessments; the results imply observational signatures across optical/UV and X-ray bands and offer explanations for anomalous TDE-like flares and delayed X-ray peaks. The findings have implications for probing IMBHs in NSCs, constraining sBH populations, and connecting transient electromagnetic signals to the dynamical state of dense galactic nuclei, with potential multi-messenger inferences via LISA from the associated EMRIs. Future work should include radiation transport, broader stellar properties, and integration into NSC dynamical models to predict TDEE rates and demographics for upcoming time-domain and gravitational-wave surveys.

Abstract

We present hydrodynamic simulations with the moving-mesh code AREPO of Tidal Disruption Encores (TDEEs) in nuclear star clusters (NSCs). TDEEs arise when a stellar-mass black hole (sBH) disrupts a star within the NSC, producing debris that is unbound from the sBH but remains gravitationally bound to the central massive black hole (MBH), leading to a delayed secondary flare. We find that the morphology and thermodynamics of the fallback material depend sensitively on the disruption geometry, MBH mass, and sBH-MBH separation. We identify two distinct morphological outcomes: ring encores, where debris circularize into a torus, and direct encores, where streams plunge toward the MBH, with encore luminosities peaking at times corresponding to the freefall timescale and one orbital period, respectively. Across all simulated cases, we find these events exhibit luminosities of $10^{40}-10^{42}$ erg/s with lightcurves characteristic of their morphology. Our work greatly improves the predictions of TDEE lightcurves and empowers observations to probe into NSC dynamics and sBH population while providing possible explanations for anomalous TDE-like flares.

Figures (13)

• Figure 1: Overview of the morphology types of TDEEs. (a) Initial three-body system of an MBH orbited by a sBH-star binary. (b) Star undergoes a micro-TDE onto the sBH while orbiting the MBH. Post micro-TDE, a direct encore (c1) or a debris ring (d1) ensue, with corresponding schematic lightcurves displayed in (c2) for a direct encore and (d2) for a ring.
• Figure 2: Rings are likely to form to the left of the dashed vertical lines, where $|\epsilon_\mathrm{sBH}|>\Delta\epsilon$. To the right of the lines, direct path encores are possible depending on the specific orbital configuration of the three bodies.
• Figure 3: Schematic depiction of the initial 3 body setup, not to scale: (a) Full 3-body system in the MBH rest frame; (b) Zoom on the sBH-star binary in the sBH rest frame.
• Figure 4: Density (top) and temperature (bottom) evolution in a simulation of a direct encore with $M_{\rm{MBH}}=10^{3}\,\mathrm{M}_\odot$ (black dot in the center), $m_{\rm{sBH}}=10\,\mathrm{M}_\odot$ (white dot), $R_{\rm bin}=10^{-5}\,\mathrm{pc} \simeq 2\times 10^{5}r_{\rm g}$, and $\theta=3\pi/4$. This is the highest luminosity encore we simulated, but it shares many features with other members with the same TDEE morphology. After a time $t\sim t_\mathrm{ff}\approx6\;\mathrm{days}$, the star is destroyed by the sBH, resulting in an accretion flow near the sBH and a stream falling into the MBH. After a time $t\sim 2t_{\rm ff}$, the tidal tail folds on itself, followed by more circular flows spreading even out to the distance of the sBH.
• Figure 5: Bolometric luminosity of the direct encore TDEEs for different initial debris orientations. The first peak corresponds to the micro-TDE around the sBH. Shocks corresponding to the encore begin at roughly the freefall timescale, which is marked. The first data point is manually added as $1 L_\odot$ at early times, since our virtual grid struggles to resolve an undisrupted star.