Accretion disc cooling and narrow absorption lines in the tidal disruption event AT 2019dsg

TL;DR

This study presents a comprehensive multi-wavelength follow-up of TDE AT 2019dsg, revealing accretion disc cooling, narrow absorption lines from debris streams, and evolving optical, X-ray, and radio emissions, providing insights into the event's physical processes.

Contribution

It offers the first detailed spectral analysis of AT 2019dsg, identifying debris stream absorption lines and clarifying the nature of its emission features and accretion disc cooling.

Findings

X-ray emission fades after ~125 days

Radio luminosity decays above 5.4 GHz after ~180 days

Narrow Fe lines appear after ~200 days

Abstract

We present the results of a large multi-wavelength follow-up campaign of the Tidal Disruption Event (TDE) \dsg, focusing on low to high resolution optical spectroscopy, X-ray, and radio observations. The galaxy hosts a super massive black hole of mass $\rm (5.4\pm3.2)\times10^6\,M_\odot$ and careful analysis finds no evidence for the presence of an Active Galactic Nucleus, instead the TDE host galaxy shows narrow optical emission lines that likely arise from star formation activity. The transient is luminous in the X-rays, radio, UV and optical. The X-ray emission becomes undetected after $\sim$125 days, and the radio luminosity density starts to decay at frequencies above 5.4 GHz by $\sim$180 days. Optical emission line signatures of the TDE are present up to $\sim$250 days after the discovery of the transient. The medium to high resolution spectra show traces of absorption lines that we propose originate in the self-gravitating debris streams. At late times, after $\sim$200 days, narrow Fe lines appear in the spectra. The TDE was previously classified as N-strong, but after careful subtraction of the host galaxy's stellar contribution, we find no evidence for these N lines in the TDE spectrum, even though O Bowen lines are detected. The observed properties of the X-ray emission are fully consistent with the detection of the inner regions of a cooling accretion disc. The optical and radio properties are consistent with this central engine seen at a low inclination (i.e., seen from the poles).