[Paper Review] "Circularization" vs. Accretion -- What Powers Tidal Disruption Events?

Tsvi Piran, Gilad Svirski+3 more|arXiv (Cornell University)|Feb 20, 2015

High-Velocity Impact and Material Behavior2 references6 citations

TL;DR

This paper proposes that the optical luminosity in tidal disruption events (TDEs) is powered by energy dissipation during the circularization of tidal debris, not by accretion onto the black hole. Simulations show circularization occurs at ~10^15 cm from the black hole, releasing ~10^44 erg/s, which better explains observed TDE light curves, temperatures, radii, and line widths than standard accretion models.

ABSTRACT

A tidal disruption event (TDE) takes place when a star passes near enough to a massive black hole to be disrupted. About half the star's matter is given elliptical trajectories with large apocenter distances, the other half is unbound. To "circularize", i.e., to form an accretion flow, the bound matter must lose a significant amount of energy, with the actual amount depending on the characteristic scale of the flow measured in units of the black hole's gravitational radius ($\sim 10^{51} (R/1000R_g)^{-1}$~erg). Recent numerical simulations \citep{Shiokawa+2015} have revealed that the circularization scale is close to the scale of the most-bound initial orbits, $\sim 10^3 M_{BH,6.5}^{-2/3} R_g \sim 10^{15} M_{BH,6.5}^{1/3}$~cm from the black hole, and the corresponding circularization energy dissipation rate is $\sim 10^{44} M_{BH,6.5}^{-1/6}$~erg/s. We suggest that the energy liberated during circularization, rather then energy liberated by accretion onto the black hole, powers the observed optical TDE candidates. The observed rise times, luminosities, temperatures, emission radii, and line widths seen in these TDEs \citep[e.g.][]{Arcavi+2014} are all more readily explained in terms of heating associated with circularization than in terms of accretion.

Motivation & Objective

• To resolve discrepancies between observed optical TDEs and classical accretion-based TDE models.
• To investigate why observed TDEs have lower luminosities, cooler temperatures, and larger emission radii than predicted by standard accretion models.
• To test whether energy dissipation during tidal debris circularization can explain the observed optical light curves and spectral features.
• To challenge the assumption that accretion onto the black hole is the primary energy source in optical TDEs.
• To provide a physically consistent explanation for the observed rise times, luminosities, and kinematics in optical TDE candidates.

Proposed method

• Uses numerical simulations (Shiokawa et al., 2015) to determine the circularization scale of tidal debris from disrupted stars.
• Calculates the energy dissipation rate during circularization using the characteristic scale of the most-bound orbits, ~10^15 M_{BH,6.5}^{1/3} cm.
• Compares predicted luminosity, temperature, emission radius, and line width from circularization heating to observed TDE properties.
• Analyzes the circularization energy dissipation rate, ~10^44 M_{BH,6.5}^{-1/6} erg/s, as the dominant energy source.
• Evaluates competing models (photon trapping, super-Eddington winds, line-driven outflows) and finds them inconsistent with observations.
• Applies observational diagnostics such as luminosity decay, temperature, and line broadening to test model predictions.

Experimental results

Research questions

• RQ1 Why do observed optical TDEs have lower luminosities and cooler temperatures than predicted by standard accretion models?
• RQ2 Why is the inferred emission radius in optical TDEs much larger (~10^15 cm) than expected for an accretion disk?
• RQ3 Why are the observed line widths (~10,000 km/s) significantly smaller than those expected for an accretion disk at R_T?
• RQ4 Can energy dissipation during circularization of tidal debris explain the observed light curves and spectral features?
• RQ5 What mechanism could produce the observed optical TDE energetics without requiring super-Eddington accretion or extreme extinction?

Key findings

• The circularization scale of tidal debris is ~10^15 M_{BH,6.5}^{1/3} cm, close to the most-bound orbit radius, with a corresponding energy dissipation rate of ~10^44 M_{BH,6.5}^{-1/6} erg/s.
• The luminosity from circularization heating is sufficient to power the observed optical TDEs, with peak luminosity ~10^43–10^44 erg/s, matching observations.
• The effective temperature of ~20,000–30,000 K and emission radius ~10^15 cm from circularization are consistent with observed TDE properties.
• The observed line broadening (~10,000 km/s) is consistent with circularization-scale dynamics, not with the much broader lines expected from an accretion disk at R_T.
• Standard accretion models fail to explain the data: predicted luminosities are too high, temperatures too hot, and emission radii too small.
• Alternative models such as photon trapping, super-Eddington winds, and line-driven outflows are ruled out due to inconsistencies with observed light curves and spectral properties.