Are there black holes in quasars?

Abstract

There are two main theoretical models for the central engine of quasars and other active galactic nuclei: the black hole model and the spinar model. Observations of the time scales of flux variations can rule out the black hole model and are therefore extremely important. The widely accepted argument of Elliot and Shapiro1 states that the maximal luminosity of the nucleus is given by the Eddington limit, LEdd = 1038 (M/M⊙) ergs−1, and the minimal time scale for periodicity is given by tmin = (gravitational radius)/(velocity of light) = 10−5 (M/M⊙) s. Therefore, the observed time scale of variations t, and the luminosity L must obey log t >log L −43. Variations on a time scale shorter than that would rule out the black hole model. We point out here that this statement is wrong in view of recent developments in the accretion disk theory: much shorter periodicities are still consistent with the black hole model.