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 Gamma = 1.92(-0.17-0.06)(+0.13+0.04) where the first and second errors represent the statistical and systematic ones. The agreement with the synchrotron radio photon index, Gamma(R) = 1.95 +/- 0.01 in 326-608.5 MHz, ensures that any excess X-ray emission is attributed to inverse Compton emission from the synchrotron-radiating electrons, boosting up the cosmic microwave background photons. From the X-ray flux density, 51.5 +/- 3.91(-5.4)(+6.2) nJy at 1 keV, derived with the photon index fixed at Gamma(R), in comparison with the synchrotron radio intensity of 10.30 +/- 0.12 Jy at 326 MHz, the magnetic and electron energy densities were estimated as u(m) = (3.0 +/- 0.2 +/- 0.4) x 10(-14)erg cm(-3) and u(e) =(34(-0.2-0.4)(+0.3+0.5)) x 10(-14)erg cm(-3) 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 tie, among all of the X-ray detected lobes of radio galaxies. A comparison of the energetics in the giant radio galaxies with a size of 1 Mpc to those in the smaller objects suggests a possibility that radio galaxies lose their jet power as they evolve from similar to 100 kpc to similar to 1 Mpc.