Tidal disruption of stars and extreme phenomena around compact objects

This thesis is the result of three years of studying the exotic transients called Tidal Disruption Events (TDEs). TDEs happen when a star gets tidally disrupted by the immense gravitational field of a supermassive black hole (SMBH). Such SMBHs reside in the nuclei of galaxies but despite their importance to their respective hosts, only a small fraction of them can be indirectly studied during their active phase known as Active Galactic Nucleus (AGN). However, most of them are quiescent/dormant, making them invisible in the electromagnetic spectrum. TDE transient flares provide a unique way to study the properties of these dormant giants and probe their demographics. However, the physical processes governing TDEs are not fully understood yet and various scenarios for the emission mechanism have been proposed. In this thesis, I study TDEs mostly in the optical and ultraviolet (UV) regime, through different astrophysical techniques like  photometry, spectroscopy and polarimetry, aiming to provide answers to those open questions.

In the first part of this thesis, I present the first detailed spectroscopic sample study of 16 optical-UV TDEs. After performing a careful and systematic reduction and analysis of the available data, I quantify their spectral properties and study them as a class. I discover a time lag between the peaks of the optical light-curves and the peak luminosity of Hα spanning between ∼ 7 – 45 days as well as a linear relationship between the Hα luminosity and the photospheric blackbody radii of TDEs. I also show that N iii strong TDEs have narrower Hα lines compared to the rest of the sample, suggesting that line widths are not driven by the kinematics. Based on this study, I suggest that the large spectroscopic diversity of TDEs combined with their X-ray properties can potentially be attributed to viewing angle effects.

The second part of the thesis is a modeling study concerning polarization of TDEs. I model the continuum polarization levels of TDEs using the 3-D Monte Carlo radiative transfer code possis and the collision-induced outflow (CIO) TDE emission scenario. Studying two different mass outflow rate cases for a variety of geometrical parameters, I find that the polarization depends strongly on the optical depths at the central regions (around the intersection point where the CIO is launched from) and the viewing angle. By comparing the model predictions to polarization observations of TDEs, I attempt to constrain their observed viewing angles and I show that multi-epoch polarimetric observations can be critical in the future in order to constrain the observed viewing angle of TDEs.

In the third part of this thesis, I present the study of AT 2020wey, a  member of the faint and fast TDE population. TDEs are typically considered to be bright with peak absolute magnitudes of the order of −20 ≲ M ≲ −19. AT 2020wey reached a peak brightness of Mg = −17.45 mag followed by a very rapid decline, making it one of the few observed faint and fast TDEs. After carefully studying its individual properties, I investigate the nature of faint and fast TDEs and, after performing a volumetric correction on a sample of 30 TDEs, I conclude that faint TDEs are not intrinsically rare and that they should constitute up to ∼ 50 - 60 % of the entire population.