New Results from X-ray Timing of TDEs

The question of how supermassive black holes (SMBHs) evolve is a fundamental mystery in high-energy astrophysics. One excellent way to address this puzzle is by constructing distributions of SMBH spins at different redshifts. This is because spin encodes evolutionary history of a black hole: higher spin implies growth by prolonged accretion while a lower spin implies evolution via mergers. However, measuring SMBH spins has been challenging because the effects of spin predicted by general relativity only emerge in the immediate vicinity of SMBHs, typically within a few gravitational radii. More recently, a new class of X-ray transients have been observed in which dormant SMBHs at centers of external galaxies rip apart stars that come too close to them. The X-rays during such tidal disruption events (TDEs) originate from strong gravity regime in the immediate vicinity of these black holes and thus encode the information about the black hole's mass and spin. I will review the recent discoveries of X-ray quasi-periodic oscillations (QPOs) and how they have been used to constrain (and measure) spins of SMBHs. I will also discuss efforts to discover similar signals/QPOs in other TDEs using current X-ray missions including Chandra, NICER, NuSTAR and XMM-Newton in the coming years, and the prospects for detecting cosmological SMBH QPOs (upto redshifts of 2 for non-jetted and ~8 for jetted TDEs) with meter-class X-ray observatories like STROBE-X and ATHENA in the next decade.