Only around 10% of the supermassive black holes (SMBHs) are actively accreting gas and thereby visible to us by the radiation of extremely hot gas (see Black Holes and X-ray Reverberation). This means the majority of SMBHs lie dormant in their galaxy centre. Tidal disruption events (TDEs) are some of the coolest ways to detect these otherwise dormant and non-detectable SMBHs and test if they are similar to their active and visible counterpart.

Tidal Disruption Event

A tidal disruption event (TDE) occurs when a star comes too close to a supermassive black hole (SMBH), causing the tidal force of the SMBH to overpower the star’s self-gravity. This results in the star being stretched as it approaches the SMBH and ultimately fully disrupted into stellar debris near its pericentre. Roughly half of the debris is gravitationally bound to the SMBH and can eventually accrete onto the SMBH, while the other half is ejected at high velocities and never returns. Such an event generates a bright flare that can last months to years, emitting radiation often across the electromagnetic spectrum, from X-rays to radio waves (Rees, 1988; Phinney, 1989). However, TDEs are rare events, happening only once every 10,000 to 100,000 years per galaxy (Magorrian & Tremaine, 1999; Wang & Merritt, 2004; Stone & Metzger, 2016).

Due to their rare and short-lived nature, TDEs are ideal targets for all-sky transient surveys, most of which are currently conducted using optical telescopes, such as the All Sky Automated Survey for SuperNovae (ASASSN), the Palomar Transient Factory (PTF) / Zwicky Transient Facility (ZTF) (Holoien et al., 2014; Hung et al., 2017; vanVelzenetal., 2021; Hammersteinet al., 2023). These surveys have different sensitivities and capabilities, which make them complementary in the search for TDEs across the electromagnetic spectrum. Also, X-ray telescopes such as ROSAT and the Swift Gamma-Ray Burst Explorer have detected several TDEs which are strong in X-rays and gamma-rays (e.g. Bade et al., 1996; Komossa & Bade, 1999; Bloom et al., 2011). One especially intriguing aspect of TDEs is that they make otherwise quiescent SMBHs detectable, allowing researchers to study the properties and demographics of these massive objects (Rees, 1988). Additionally, TDEs can help detect intermediate-mass black holes (IMBHs) that may reside in dwarf galaxies and stellar clusters. Furthermore, TDEs can provide insight into the population and dynamics of stars in the innermost regions of galaxies. Finally, TDEs are a promising avenue for studying the physics of accretion disks and jets around black holes. In this section, I present some of the basic theories and observational facts of TDEs. Then, I describe how the modelling of TDEs has evolved over the past decade. It is worth mentioning that even the most sophisticated models developed so far require complex calculations conducted on supercomputers. However, as I show, more detailed modelling is still needed to comprehend all the physical processes happening in TDEs.

Early TDE studies often assume that the debris rapidly forms an accretion disk and accretes on timescales shorter than the fallback time. If this were true, the accretion rate would follow the fallback rate, indicating that the accretion process is super-Eddington. This would suggest that a super-Eddington accretion disk forms and evolves into a sub-Eddington flow typically within months to years, underscoring the significance of TDEsasaplatformtotestaccretion physics, particularly in relation to super-Eddington accretion. Nonetheless, the rapid formation of a disk is still an active research topic and is further discussed in Section 1.3.3. Rees (1988) and Phinney (1989) proposed that TDEs are powered by emission from a super-Eddington accretion disk. They adopted the thin disk model and predicted that TDEs are expected to be X-ray bright with temperatures ranging from 105 to 106 K, so they are best observed using X-ray telescopes in space. Assuming a constant radiative efficiency throughout the evolution of a TDE, the luminosity produced in the accretion disk can be described by the equation: LFB = η ˙ MFBc2 (1.23) Therefore, the light curve of a TDE is expected to follow the evolution of the debris mass fallback rate. By fitting the observed luminosity as a function of time, it is then possible to estimate the black hole mass and the parameters of the disrupted star as they set the fallback rate. Examples of TDE light curves are shown in the lower right panel of Fig. 1.6. It is important to note that pre-peak and near-peak observations of TDEs play a significant role in unravelling some of the inherent model degeneracies when estimating the masses of the black hole and the disrupted star. Many current fitting models assume that the light curves strictly follow the fallback rate at early times, which is not always the case (Mockler et al., 2019a).