Contribution of Stellar Tidal Disruptions to the X‐Ray Luminosity Function of Active Galaxies

The luminosity function of active galactic nuclei has been measured down toluminosities ~10^{42} erg/s in the soft and hard X-rays. Some fraction of thisactivity is associated with the accretion of the material liberated by thetidal disruption of stars by massive black holes. We estimate the contributionto the X-ray luminosity function from the tidal disruption process. While thecontribution depends on a number poorly known parameters, it appears that itcan account for the majority of X-ray selected AGN with soft or hard X-rayluminosities <~10^{43}-10^{44} erg/s. If this is correct, a picture emerges inwhich a significant portion of the X-ray luminosity function of AGN iscomprised of sources powered by tidal-disruption at the faint end, while thesources at the bright end are powered by non-stellar accretion. Black holeswith masses <~2x10^6 Msun could have acquired most of their present mass by anaccretion of tidal debris. In view of the considerable theoretical uncertaintyconcerning the detailed shape of the light curves of tidal disruption events,we focus on power-law luminosity decay (as identified in candidate tidaldisruption events), but we also discuss constant accretion rate models.Comment: 5 pages, 1 figur