A Simple Test for the Existence of Two Accretion Modes in Active Galactic Nuclei

By analogy to the different accretion states observed in black hole X-ray binaries (BHXBs), it appears plausible that accretion disks in active galactic nuclei (AGNs) undergo a state transition between a radiatively efficient and inefficient accretion flow. If the radiative efficiency changes at some critical accretion rate, there will be a change in the distribution of black hole masses and bolometric luminosities at the corresponding transition luminosity. To test this prediction, I consider the joint distribution of AGN black hole masses and bolometric luminosities for a sample taken from the literature. The small number of objects with low Eddington-scaled accretion rates ṁ < 0.01 and black hole masses MBH < 109 M☉ constitutes tentative evidence for the existence of such a transition in AGNs. Selection effects, in particular those associated with flux-limited samples, systematically exclude objects in particular regions of the (MBH, Lbol) plane. Therefore, they require particular attention in the analysis of distributions of black hole mass, bolometric luminosity, and derived quantities such as the accretion rate. I suggest further observational tests of the BHXB-AGN unification scheme that are based on the jet domination of the energy output of BHXBs in the hard state, and of the possible equivalence of BHXB in the very high (or steep power-law) state showing ejections and efficiently accreting quasars and radio galaxies with powerful radio jets.