Constraints on the average magnetic field strength in a relic radio galaxy derived from limits on inverse Compton X-rays

We have observed the relic radio galaxy, 0917+75, for 15 ks with the Position Sensitive Proportional Counter (PSPC) of the ROSAT X-ray satellite. Although the source was not detected, our upper limit for the X-ray flux enables us to estimate a lower limit to the average magnetic field strength in the source of ~=1 muG (0.1 nT). If the minimum energy condition holds for relativistic particles and magnetic field, and if the inferred power law of relativistic electrons extends a factor of 3 below the energy responsible for the observed 20-MHz emission, then we demonstrate that the energy density in relativistic electrons which produces the observed radio emission must be augmented with a substantial contribution from lower energy electrons or relativistic protons.