TheXMM‐NewtonDetection 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 isreported. In two exposures on the target, faint diffuse X-ray emissionassociated with the radio lobes was significantly detected, together with abright X-ray active nucleus, of which the 2 -- 10 keV intrinsic luminosity is(4 -- 8) \times 10^{42} erg s-1. The EPIC spectra of the northern and southernlobes are reproduced by a single power law model modified by the Galacticabsorption, with a photon index of 2.2-0.5+0.6 and 1.7-0.6+0.7 respectively.These indices are consistent with that of the radio synchrotron spectrum, 1.73+- 0.01 The luminosity of the northern and southern lobes are measured to be8.3-2.6+3.3 \times 10^{40} erg s-1 and 9.2-4.3+5.7 \times 10^{40} erg s-1,respectively, in the 0.7 -- 7 keV range. The diffuse X-ray emission isinterpreted as an inverse-Compton emission, produced when thesynchrotron-emitting energetic electrons in the lobes scatter off the cosmicmicrowave background photons. The magnetic field in the lobes is calculated tobe about 1.7 \mu G, which is about 2.5 times lower than the value estimatedunder the minimum energy condition. The energy density of the electrons isinferred to exceed that in the magnetic fields by a factor of 40 -- 50.Comment: 23 pages, 7 figures. Accepted for publication in the Astrophysical Journa