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

Abstract

The luminosity function of active galactic nuclei (AGNs) has been measured down to luminosities ~10^(42) ergs s^(-1) in the soft and hard X-rays. Some fraction of this activity is associated with the accretion of the material liberated by the tidal disruption of stars by massive black holes. We estimate the contribution to the X-ray luminosity function from the tidal disruption process. While the contribution depends on a number of poorly known parameters, it appears that it can account for the majority of X-ray-selected AGNs with soft or hard X-ray luminosities ≲10^(43)-10^(44) ergs s^(-1). If this is correct, a significant portion of the X-ray luminosity function of AGNs is comprised of sources powered by tidal disruption at the faint end, while the sources at the bright end are powered by nonstellar accretion. Black holes with masses ≲2 × 10^6 M_☉ could have acquired most of their present mass by an accretion of tidal debris. In view of the considerable theoretical uncertainty concerning the detailed shape of the light curves of tidal disruption events, we focus on power-law luminosity decay (as identified in candidate tidal disruption events), but we also discuss constant accretion rate models. We expect that the sources associated with thin-disk accretion of circularized tidal debris have peak luminosities of ≲10^(39)-10^(41) ergs s^(-1), below the luminosity range probed in present surveys.