In the hydrodynamic simulations of RHB, we identified three evolutionary phases. Early on, the cocoon is highly overpressured, driving a strong shock into the ICM. Simulated X-ray maps show a cavity surrounded by a thin, hot shell. Once the cocoon comes into pressure equilibrium with the ICM, the sideways expansion of the cocoon becomes sub-sonic and the shock becomes a compression wave, though active jets can keep the advance speed of the jet heads supersonic for much longer, driving ``sonic booms'' into the ICM. For roughly a sound crossing time, the well defined cavity created during the supersonic phase will survive and be observable. Then, hydrodynamic instabilities will destroy the cavity altogether.
A strong shock would show up as a sharp feature in our X-ray images, which have an effective resolution2 of 4-5arcsec at the edges of the cavities. The absence of such a shock indicates that the source expansion is no longer highly supersonic. We identify the hourglass-like feature in the core with the compression wave (``sonic boom'') found in our hydrodynamic simulations (Fig. 1, RHB; in our simulated X-ray maps the brightest emission also tends to be in the equatorial plane).