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Suzaku measurement of electron and magnetic energy densities in the east lobe of the giant radio galaxy DA 240

A careful analysis of the Suzaku data of the giant radio galaxy DA 240, of which the size is 1.48 Mpc, revealed diffuse X-ray emission associated with its east lobe. The diffuse X-ray spectrum was described with a simple power-law model with a photon index of 1.92 (-0.17+0.13) (-0.06+0.04), where the first and second errors represent the statistical and systematic ones. The agreement with the synchrotron radio photon index, 1.95 +- 0.01 in 326 -- 608.5 MHz, ensures that the excess X-ray emission is attributed to the inverse Compton emission from the synchrotron-radiating electrons, boosting up the cosmic microwave background photons. From the X-ray flux density, 51.5 +-3.9 (-5.4+6.2) nJy at 1 keV derived with the photon index fixed at the radio index, in comparison with the synchrotron radio intensity of 10.30 +- 0.12 Jy at 326 MHz, the magnetic and electron energy densities was estimated as u_m = (3.0 +-0.2 +-0.4) \times 10^(-14) ergs/cm3 and u_e = (3.4 (-0.2+0.3) (-0.4+0.5)) \times 10^(-14) ergs/cm3 integrated over the electron Lorentz factor of 10^3 -- 10^5, respectively. Thus, the east lobe is found to reside in an equipartition condition between the electrons and magnetic field parametrized as u_e/u_m = 1.1 (-0.1+0.2) (-0.2+0.4). The east lobe of DA 240 is indicated to exhibit the lowest value of u_e, among all the X-ray detected lobes of radio galaxies. A comparison of the energetics in the giant radio galaxies with a size of about 1 Mpc to those in the smaller objects suggests a possibility that radio galaxies lose their jet power, as they evolve from 100 kpc to 1 Mpc.