[Paper Review] Radiative Emission Mechanisms of Tidal Disruption Events

Nathaniel Roth, Elena M. Rossi+4 more|arXiv (Cornell University)|Aug 3, 2020

Astrophysical Phenomena and ObservationsPhysics and Astronomy169 references49 citations

TL;DR

This paper investigates radiative emission mechanisms in tidal disruption events (TDEs), proposing that optical/UV and soft X-ray emissions arise from either rapidly circularized accretion discs with modified radiation due to outflows, reprocessing, and Comptonization, or from shock-heated, eccentric debris streams. The key contribution is a unified explanation for the near-constant UV/optical color temperatures via incomplete thermalization in the debris atmosphere, reconciling observed multi-band light curves with theoretical models.

ABSTRACT

We describe how the various outcomes of stellar tidal disruption give rise to observable radiation. We separately consider the cases where gas circularizes rapidly into an accretion disc, as well as the case when shocked debris streams provide the observable emission without having fully circularized. For the rapid circularization case, we describe how outflows, absorption by reprocessing layers, and Comptonization can cause the observed radiation to depart from that of a bare disc, possibly giving rise to the observed optical/UV emission along with soft X-rays from the disc. If, instead, most of the debris follows highly eccentric orbits for a significant time, many properties of the observed optical/UV emission can be explained by the scale of those eccentric orbits and the shocks embedded in the debris flow near orbital apocenter. In this picture, soft X-ray emission at early times results from the smaller amount of debris mass deflected into a compact accretion disc by weak shocks near the stellar pericenter. A general proposal for the near-constancy of the ultraviolet/optical color temperatures is provided, by linking it to incomplete thermalization of radiation in the atmosphere of the emitting region. We also briefly discuss the radio signals from the interaction of unbound debris and jets with the black hole environment.

Motivation & Objective

• To explain the observed optical/UV and soft X-ray emission in tidal disruption events beyond the standard bare accretion disc model.
• To address the discrepancy between theoretical soft X-ray predictions and observational limits at early times.
• To account for the near-constant UV/optical color temperatures observed in TDE light curves.
• To model radio emission from unbound debris and relativistic jets interacting with the circum-nuclear medium.
• To unify multiple emission mechanisms under a single framework that explains multi-band light curves.

Proposed method

• Models accretion disc emission with initial mass fallback rates and later viscous evolution, comparing to observed X-ray and UV data.
• Introduces outflows, reprocessing layers, and Comptonization as mechanisms that modify bare disc emission, shifting peak flux from soft X-rays to UV/optical.
• Analyzes shock-heated, eccentric debris streams, linking their orbital scale and apocenter shocks to early optical/UV emission.
• Uses analytical and semi-analytical models for synchrotron radio emission from unbound debris and relativistic jets, based on shock acceleration and energy partition.
• Applies equipartition and blast wave models to radio afterglow evolution, incorporating time-dependent energy input from inverse Compton cooling.
• Proposes a dynamic photosphere model tied to mass fallback rate, allowing for evolving spectral energy distributions.

Experimental results

Research questions

• RQ1 How do outflows, reprocessing, and Comptonization modify the radiation from a bare accretion disc in TDEs?
• RQ2 Why is the UV/optical color temperature nearly constant during the early evolution of TDEs?
• RQ3 What physical processes in eccentric, shock-heated debris streams produce the observed optical/UV emission?
• RQ4 How does radio emission from unbound debris and relativistic jets evolve over time in TDEs?
• RQ5 What explains the time-dependent energy input in radio afterglows of TDEs like Swift J1644?

Key findings

• The observed optical/UV emission in TDEs is better explained by reprocessed and Comptonized radiation from accretion discs rather than bare disc emission, which overpredicts soft X-rays.
• Near-constant UV/optical color temperatures arise from incomplete thermalization in the atmosphere of shock-heated debris, not from a stable photosphere.
• Soft X-ray emission at early times results from a small fraction of debris mass deflected into a compact disc via weak shocks near pericenter.
• Radio emission from unbound debris can be prompt and detectable, with characteristic speeds of ~6000 km/s and kinetic energies ~2×10^50 erg for M* ~1 M⊙.
• Radio afterglows in jetted TDEs like Swift J1644 show increasing energy in the emitting region over time, explained by decreasing X-ray flux reducing inverse Compton cooling of electrons.
• The model successfully fits multi-band TDE light curves by linking them to the post-disruption mass fallback rate and a dynamically adjusting photosphere.