  We argue that the observed `accretion rate gap' - between black holes in
radio-loud active galactic nuclei (AGN) accreting at close to the Eddington
limit and those accreting at considerably lower rates - can be explained in
terms of the adiabatic inflow-outflow (ADIOS) scenario for radiatively
inefficient accretion. Whenever the accretion rate falls below a threshold
value (corresponding to a luminosity L_crit) that depends on the viscosity
parameter, alpha, the inner region of the accretion disk - extending from the
marginally stable orbit to ~ 1000 Schwarzschild radii - is susceptible to
becoming hot and radiatively inefficient. If this happens, the disk luminosity
decreases by a factor of ~100, as most of the matter originally destined to be
swallowed is instead expelled in a wind. According to our conjecture, accretion
flows onto black holes never radiate steadily in the range ~ 0.01 L_crit < L <
L_crit, hence the inferred accretion rate gap. We expect the gap to exist also
for black holes in X-ray binaries, where it may be responsible for state
transitions and the luminosity fluctuations associated with X-ray nova
outbursts.
