Measuring black hole spins in TDEs: a QPO in ASASSN-14 li
In this talk I am going to discuss the possibility to observe rigid Lense-Thirring precession of an accretion discs around a supermassive black hole. The debris of a tidally disrupted star are expected to form a thick and narrow accretion disc around the supermassive black hole. Since the initial star orbit is in general inclined with respect to the black hole spin, this misalignment combined with the Lense-Thirring effect leads to a warp in the disc. This warped disc can precess as a rigid body around the black hole and aligns with its spin when the warp is dissipated. The black hole spin and mass value can be inferred by computing the precession period and the alignment timescale. The light curves of these events sometimes show a quasi-periodic modulation of the flux that can be associated with the rigid precession period of the accretion disc due to the Lense-Thirring effect. The discovery of a remarkably loud and exceptionally stable quasi-periodic oscillation (QPO) detected in the spectrum of the candidate TDE ASASSN-14li provides an opportunity to test our model and thus to obtain information about the fundamental properties of the black hole in the host galaxy.