Stellar Autopsies: Shredded Evidence from Galactic Centres

Abstract: When a star wanders too close to a supermassive black hole (SMBH) lurking in a galaxy’s nucleus, it meets an unfortunate end: shredded apart by the black hole’s gravitational field, its remains fuel a luminous, panchromatic transient flare — a tidal disruption event (TDE). By their very nature, TDEs are a unique tool for probing SMBHs and their existence was predicted by theorists since the 80s. The first candidates were discovered in the X-ray regime in the late 90s and only at 2006 the detections of potential candidates in the optical and UV began. Since then, thanks to the rapid development of dedicated large surveys for time-domain astronomy, the number of detected candidates has surpassed 100. However, even though TDEs are now routinely discovered by surveys, the physics underlying the emission mechanism that powers them has remained elusive.
 
Spectroscopically, the first optical spectrum of a TDE was obtained in 2007; it displayed broad emission features around Hα, Hβ and He II λ4686 and, to-date, these remain  the most characteristic spectral features for a nuclear transient to be classified as a TDE. Nevertheless, since then we have come to understand that the spectral properties of optical/UV TDEs show a large diversity, potential evolution with time and, occasionally, unique features that are puzzling and challenging to understand.
 
In this talk, I will attempt to give a concise review of TDEs and their spectral properties in the optical/UV, and discuss potential links between those properties and the geometrical structures, as well as their (still) elusive emission mechanism.