On the observational evidence for accreting black holes in quasars

Abstract

We review (mainly high energy) observations which could lend support to (or dismiss) the hypothesis of an accreting central black hole in quasars. Direct imaging is not powerful (1). We focus on three main topics: 2) variability as a constraint of the quasar compactness; 3) X-ray continuum; 4) X-ray spectral features, expected from reprocessing of the X-ray radiation from matter near the X-ray source. We argue that the above observations provide a weaker evidence than once thought for a black hole as the engine of quasars. New tests will come from (5) the study of the evolution of the quasar Spectral Energy Distribution (SED) in the framework of models of quasar physical evolution. We present some new results, obtained comparing ROSAT X-ray observations of z=2–3 quasars with previous monitoring of low-z quasars, which represent a first steep in this direction.