ROSAT observations of 3C 388: A test of minimum energy

We have made X-ray observations of the radio galaxy 3C 388 with the ROSAT HRI, resolving the thermal gas that confines the lobes of this powerful (FR II) radio source. The gas pressure is substantially higher than the minimum pressures in the radio lobes, implying that the latter is an underestimate by an order of magnitude or more. We discuss how this can be reconciled with recent observations that find inverse-Compton emission from radio lobes at the level predicted from conventional minimum-energy arguments. We suggest that the excess pressure is due to low-energy electrons and positrons and/or protons, so that magnetic field strengths are below equipartition. If this calibration of minimum-energy arguments applies to radio lobes in general, the power of radio jets has been underestimated by the same factor, implying that jet kinetic energy flux is close to or exceeds bolometric luminosity in radio-loud active galactic nuclei (AGNs). Most of the X-rays from 3C 388 are emitted by the thermal gas, which is well centered on the host cD galaxy, itself the center of a small cluster. This component is well fitted by a standard β model. The ends of the radio lobes are beyond the ≈33 kpc core radius, so the density is declining roughly as a power law with ρ ∝ R -1.6. On most models, this is not a fast enough decline to allow a constant expansion speed for the source. A compact X-ray source with 0.1-2.4 keV luminosity of 5 x 1035 W lies at the center. This is probably the AGN, although a galaxy-scale "cooling-flow" core cannot be ruled out.