| Abstract: | Supermassive black holes (SMBHs) are ubiquitous in the nuclei of massive galaxies, and are central to many aspects of galaxy evolution and high energy astrophysics. They also appear to be astrophysical manifestations of the Kerr metric, and thus fully characterizable by just two numbers: their mass and spin angular momentum. Despite the importance of SMBHs for astrophysics, their demographics are still poorly understood, as are their origins and growth history. The unknown bottom end of the SMBH mass function (do SMBHs and/or intermediate mass black holes exist in dwarf galaxies?) contains valuable information on the exotic high-redshift origins of SMBHs, while the generally unknown SMBH spin function (only about 30 SMBH spins have been constrained from observations) likewise encodes their growth histories. I will discuss the use of a relatively novel electromagnetic probe of SMBH mass and spin: tidal disruption events (TDEs). TDEs are transient accretion flares produced when stars pass too close to massive black holes, are torn apart by tides, and begin dissipating orbital energy through shocks or accretion processes. I will review the ways in which SMBH properties are encoded in the light curves and spectra of TDEs, with an emphasis on the existing theoretical uncertainties. I will then focus more specifically on our recent work fitting relativistic disk models to the continuum X-ray flux of TDEs. We have used this approach has to produce mass and spin constraints on 3 different massive black holes, including a rapidly spinning intermediate mass black hole between one thousand and ten thousand Solar masses in size. The measurement of intermediate mass black hole spins constrains a unique mass range in ultralight bosonic dark matter candidates, as I will discuss. |