A common feature of the X-ray-bright shells surrounding the
radio bubbles shown in Section 2 is that the pressure measured
for the shells is approximately equal to that just outside of
them. In other words, there is no evidence for a strong shock.
Another feature common to many of these objects is that the
pressure measured in the X-ray-bright shells is about an order
of magnitude higher than the pressure measured from the radio
data within the radio-bright bubbles, assuming equipartition of
energies (an example is Abell 2052, with an X-ray shell pressure
of 1.5 × 10-10 dyn cm-2 [Blanton et al. 2001], and a radio
equipartition pressure of 2 × 10-11 dyn cm-2 [Zhao et al.
1993]). However, we expect that the bubbles and the shells are
in pressure equilibrium, since otherwise they would collapse and
fill in. Therefore, either some of the assumptions made for the
equipartition pressure estimates are incorrect, or there is an
additional source of pressure within the radio bubbles. This
additional pressure component