  We show that the time delay between the optical and X-ray outbursts of the
black-hole soft X-ray transient source GRO J1655-40, observed in April 1996,
requires that the accretion flow in this object must consist of two components:
a cold outer accretion disk and an extremely hot inner advection-dominated
accretion flow (ADAF). In quiescence, the model predicts a spectrum which is in
good agreement with observations, with most of the observed radiation coming
from the ADAF. By fitting the observed spectrum, we estimate the mass accretion
rate of the quiescent system and the transition radius between the disk and the
ADAF. We present a detailed numerical simulation of a dwarf-nova type
instability in the outer disk. The resulting heat front reaches the ADAF cavity
promptly; however, it must then propagate inward slowly on a viscous time
scale, thereby delaying the onset of the X-ray flux. The model reproduces the
observed optical and X-ray light curves of the April 1996 outburst, as well as
the 6-day X-ray delay. Further, the model gives an independent estimate of the
quiescent mass accretion rate which is in very good agreement with the rate
estimated from fitting the quiescent spectrum. We show that a pure thin disk
model without an ADAF zone requires significant tuning to explain the X-ray
delay; moreover, such a model does not explain the quiescent X-ray emission of
GRO J1655-40.
