  A new instability mechanism is described in accretion flows where the gas is
accelerated from a stationary shock to a sonic surface. The instability is
based on a cycle of acoustic and entropic waves in this subsonic region of the
flow. When advected adiabatically inward, entropy perturbations trigger
acoustic waves propagating outward. If a shock is present at the outer
boundary, acoustic waves reaching the shock produce new entropy perturbations,
thus creating an entropic-acoustic cycle between the shock and the sonic
surface. The interplay of acoustic and entropy perturbations is estimated
analytically using a simplified model based on the compact nozzle
approximation. According to this model, the entropic-acoustic cycle is unstable
if the sound speed at the sonic surface significantly exceeds the sound speed
immediately after the shock. The growth rate scales like the inverse of the
advection time from the outer shock to the sonic point. The frequency of the
most unstable perturbations is comparable to the refraction cutoff, defined as
the frequency below which acoustic waves propagating inward are significantly
refracted outward. This generic mechanism should occur in Bondi-Hoyle-Lyttleton
accretion, and also in shocked accretion discs.
