  A hyperaccretion flow around a stellar mass black hole is thought to be the
most plausible engine that powers gamma-ray bursts (GRBs). The flow efficiently
cools via neutrino emission at >~ 0.003-0.01 M_sun s^{-1} (corresponding to a
luminosity of ~10^50 erg s^{-1}), while neither neutrino nor photon emission is
efficient below this rate, so the flow should be advection dominated. We
carefully solve how a transition occurs from the advection-dominated to the
neutrino-dominated branches, and find that the slope of the thermal equilibrium
curve is negative in the surface density - accretion rate (Sigma-Mdot) plane, a
condition for viscous instability, at radii smaller than ~12R_g (with R_g being
the gravitational radius). We also confirm that the flow is thermally stable.
The consequence of this instability is the formation of a clumpy structure in
the flow. This is because the larger (respectively smaller) surface density is,
the smaller (respectively larger) the mass accretion rate from the region in
question becomes, leading to growth of the density contrast. The timescale for
clump formation is estimated to be shorter than 0.1s. The observational
implication is discussed in the context of GRBs. We suggest that this might
explain the origin of the large variability observed in the prompt emission of
GRBs.
