Shining lights, even in death: modelling the optical and ultraviolet emission from tidal disruption events

Abstract

When an unlucky star wanders too close to a supermassive black hole, the self-gravity keeping that star together is completely overwhelmed by the tidal forces of the interaction and is torn asunder in something known as a tidal disruption event (TDE). The fallback of the stellar debris, and subsequent accretion, onto the black hole drives a powerful, but transient, luminous flare visible across the electromagnetic spectrum: a shining light, even in death. Within the past decade, the census of TDEs has rapidly grown to a dizzying 56 events. Theoretical progress, on the other hand, has somewhat lagged behind and our understanding of these events is held back by our ignorance of the emission mechanisms fuelling the luminous flare. Given their extreme luminosities, TDEs are expected to generate significant mass loss in the form of radiation driven winds, but their properties remain poorly understood even though their importance is widely acknowledged. In this thesis, I use state-of-the-art Monte Carlo radiative transfer and ionization software to model the profound impact such powerful outflows could have on the optical and ultraviolet (UV) spectra of these events. Focusing exclusively on the sub-Eddington accretion phase, I first show how the diverse family of UV spectra, showcasing broad absorption and emission lines, can be unified as an orientation effect associated with line formation in an accretion disc wind. And if true, I suggest that the relative number of broad absorption to emission line TDEs could be used to estimate the outflow covering fraction. I also demonstrate how such outflows can efficiently reprocess thermal emission from an accretion disc, producing the broad Balmer and helium optical recombination features characteristic of TDEs, as well as a spectral energy distribution (SED) with a much lower characteristic colour temperature. I next apply the exact same numerical techniques to post-process a snapshot of a realistic hydrodynamic model of the super-Eddington accretion phase, illustrating how different reprocessing regimes can modify the SED and influence the observable signatures of TDEs. In summary, then, this thesis demonstrates how optically thick outflows impact and shape the observational properties of TDEs, and why, in the future, detailed radiative transfer calculations are required to understand the full complexities of TDE emission.

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Identifiers

ORCID for Edward, John Parkinson: orcid.org/0000-0003-3902-052X

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