& Rees 1988). The X-ray variations are more suggestive of localized flaring types of activity that may arise in a patchy corona above the accretion disk.

It has already been pointed out based on these same [*RXTE*]{} data (Uttley et al. 1999) that the X-ray continuum of NGC 4051 virtually “turned off” in early 1998 (around JD 2450800; see Fig. 1). However, the optical spectroscopic data show that the optical continuum and emission lines (and therefore, by inference, the ionizing UV continuum) did not disappear at the same time. This seems in a sense rather suggestive of the kind of behavior that has been seen in Galactic black-hole systems such as GRO J1655$-$40 (Orosz et al. 1997). A possible interpretation of the behavior of NGC 4051 is that the inner X-ray producing (though not necessarily by purely thermal emission) part of the accretion disk in NGC 4051 has entered an advection-dominated accretion-flow (ADAF) state, in which radiation is emitted with very low efficiency (Narayan & Yi 1994; Narayan et al. 1998). This implies that there is a transition radius $r_{\rm tran}$ inside of which the disk is an ADAF and outside of which it radiates efficiently, perhaps like a classical 