First, they reduce the maximal Hills mass by extracting black hole spin. In this work, we study the potential of TDE rate measurements to probe SMBH spins and test superradiant spin-down due to new bosons. Additional bounds could be set if the ULBs were to be the dark matter or to have additional non-gravitational couplings 15, 16, or by looking for gravitational wave emission from the superradiant cloud at LISA 12, or by studying X-ray TDE spectra 17. Currently, bounds on such ULBs are obtained from SMBH spin measurements 12, 13, but the robustness of these measurements is a topic of debate 14. If SMBHs have been spun down by ULBs, measurements of TDE rates could then be used to test new physics. If these particles are bosonic in nature, however, they could leave imprints on the SMBH spin distributions as a result of the superradiant instability, a purely gravitational process that creates a cloud of ultra-light bosons (ULBs) at the expense of the SMBH’s energy and angular momentum 9, 10, 11. Theoretical considerations strongly motivate the possibility that at least some of the new particles are ultra-light and weakly coupled, rendering them challenging to detect 7, 9. Evidence from cosmological and astrophysical observations suggests that nature contains new degrees of freedom that likely reside in a complex dark sector 7, 8. Additionally, an exciting possibility is that SMBH spins could be affected by new physics. The magnitude of SMBH spins is set by gas accretion and galaxy mergers, allowing TDE rate measurements to provide valuable insights into these processes. While current TDE counts give us only a limited idea of the rates at those masses 5, the Legacy Survey of Space and Time (LSST) is expected to observe as many as ~10 5 TDEs in the optical range 6, dramatically increasing the current dataset. These features can be used to probe SMBH spins using TDE rate measurements, but this requires enough statistics to sample galaxies containing black holes with masses M BH ~ 10 8 M ⊙, which is the Hills mass for the disruption of Sun-like stars. For SMBH masses close to but below the Hills cutoff the dependence of the TDE rates with spin persists, with larger spins leading to larger rates 4. Larger spins increase the Hills mass 4, an effect that can be intuitively understood from the fact that the horizon radius decreases with increasing spin. TDEs do not occur in SMBHs with masses above a critical “Hills mass” 1, since for the most massive black holes stars that lead to TDEs enter the horizon before observable disruptions can occur.

The event rates sensitively depend on the SMBH spins 4, as TDEs can occur close to the horizon where the geometry is affected by the black hole rotation. These disruptions lead to bright flares that have been observed by the dozens in optical, UV, and X-ray sky surveys 3. Stellar tidal disruption events (TDEs) take place in the center of galaxies, where stars can be ripped apart by the tidal forces induced by the gravitational potential of supermassive black holes (SMBHs) 1, 2.

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