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 may be magnetic fields, low energy relativistic electrons, or very hot, diffuse, thermal gas that would not be detected by Chandra because of its low surface brightness in the Chandra energy band. The temperature of hot, thermal gas that would provide the required pressure to support the X-ray shells has been limited to > 15 keV for Hydra A (Nulsen et al. 2002), > 11 keV for Perseus (Schmidt et al. 2002), and > 20 keV for Abell 2052 (Blanton et al. 2003). High sensitivity at high energies is necessary to detect diffuse gas at such temperatures, and XMM-Newton or the upcoming Constellation-X may be able to detect it.
A detection of gas within an X-ray depression with a temperature significantly hotter than its surroundings has been made using Chandra data of the cooling flow cluster MKW 3s (Mazzotta et al. 2002). The gas in the bubble is hotter than the gas at any radius in the cluster, and the temperature measurement is therefore not a projection effect. The deprojected gas temperature within the bubble is 7.5 keV, compared with a temperature of 3.5 - 4 keV for the surrounding emission. This cluster contains a central radio source, however the 1.4 GHz radio emission is not directly connected with the X-ray depression, as shown in Mazzotta et al. 2002.