Tidal disruption rate suppression by the event horizon of spinning black holes

Abstract

ABSTRACT The rate of observable tidal disruption events (TDEs) by the most massive (≳ few × 107 M⊙) black holes (BHs) is suppressed due to direct capture of stars. This suppression effect depends on the space–time geometry and holds the promise of probing the spin distribution of dormant BHs at the centres of galaxies. By extending the frozen-in approximation commonly used in the Newtonian limit, we propose a general relativistic criterion for the tidal disruption of a star of given interior structure. The rate suppression factor is then calculated for different BH masses, spins, and realistic stellar populations. We find that either a high-BH spin (≳ 0.5) or a young stellar population (≲1 Gyr) allows TDEs to be observed from BHs significantly more massive than $10^8\, \mathrm{M}_{\odot }$. We call this spin-age degeneracy (SAD). This limits our utility of the TDE rate to constrain the BH spin distribution, unless additional constraints on the age of the stellar population or the mass of the disrupted star can be obtained by modelling the TDE radiation or the stellar spectral energy distribution near the galactic nuclei.