Reverberation in tidal disruption events: dust echoes, coronal emission lines, multi-wavelength cross-correlations, and QPOs

TL;DR

This review discusses how reverberation signals from dust and gas in galactic nuclei following tidal disruption events (TDEs) reveal insights into the properties and geometry of these cosmic phenomena across multiple wavelengths.

Contribution

It synthesizes recent observational techniques and findings related to reverberation signals in TDEs, highlighting the potential of upcoming instruments to enhance understanding.

Findings

Infrared echoes from heated dust reveal TDE reprocessing.

Coronal emission lines exhibit reverberation, tracing gas orbiting the black hole.

X-ray observations detect QPOs and reverberation lags, providing insights into accretion dynamics.

Abstract

Stellar tidal disruption events (TDEs) are typically discovered by transient emission due to accretion or shocks of the stellar debris. Yet this luminous flare can be reprocessed by gas or dust that inhabits a galactic nucleus, resulting in multiple reverberation signals. Nuclear dust heated by the TDE will lead to an echo at infrared wavelengths (1-10 $\mu$m) and transient coronal lines in optical spectra of TDEs trace reverberation by gas that orbits the black hole. Both of these signal have been detected, here we review this rapidly developing field. We also review the results that have been extracted from TDEs with high-quality X-ray light curves: quasi periodic oscillations (QPOs), reverberation lags of fluorescence lines, and cross-correlations with emission at other wavelengths. The observational techniques that are covered in this review probe the emission from TDEs over a wide range of scales: from light years to the innermost parts of the newly formed accretion disk. They provide insights into important properties of TDEs such as their bolometric output and the geometry of the accretion flow. While reverberation signals are not detected for every TDE, we anticipate they will become more commonplace when the next generation of X-ray and infrared instruments become operational.