The XMM-Newton Detection of Diffuse Inverse Compton X-Rays from Lobes of the FR II Radio Galaxy 3C 98

The XMM-Newton observation of the nearby FR II radio galaxy 3C 98 is reported. In two exposures on the target, faint diffuse X-ray emission associated with the radio lobes was significantly detected together with a bright X-ray active nucleus, of which the 2-10 keV intrinsic luminosity is (4-8) × 1042 ergs s-1. The EPIC spectra of the northern and southern lobes are reproduced by a single power-law model modified by the Galactic absorption, with a photon index of 2.2 and 1.7, respectively. These indices are consistent with that of the radio synchrotron spectrum, 1.73 ± 0.01. The luminosities of the northern and southern lobes are measured to be 8.3 × 1040 and 9.2 × 1040 ergs s-1, respectively, in the 0.7-7 keV range. The diffuse X-ray emission is interpreted as an inverse Compton emission, produced when the synchrotron-emitting energetic electrons in the lobes scatter off the cosmic microwave background photons. The magnetic field in the lobes is calculated to be about 1.7 μG, which is about 2.5 times lower than the value estimated under the minimum energy condition. It is inferred that the energy density of the electrons exceeds that in the magnetic fields by a factor of 40-50.