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 $_{-0.17}^{+0.13}$ $_{-0.06}^{+0.04}$ , where the first and second errors represent the statistical and systematic ones. The agreement with the synchrotron radio photon index, $\Gamma_{\rm R}$ $=$ 1.95 $\ \pm\ $ 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 $\ \pm\ $ 3.9 $_{-5.4}^{+6.2}\ $ nJy at 1 keV, derived with the photon index fixed at $\Gamma_{\rm R}$ , in comparison with the synchrotron radio intensity of 10.30 $\ \pm\ $ 0.12 Jy at 326 MHz, the magnetic and electron energy densities were estimated as $u_{\rm m}$ $=$ (3.0 $\ \pm\ $ 0.2 $\ \pm\ $ 0.4) $\times$ 10 $^{-14}\ $ erg cm $^{-3}$ and $u_{\rm e}$ $=$ (3.4 $_{-0.2}^{+0.3}$ $_{-0.4}^{+0.5}$ ) $\times$ 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 $u_{\rm e}/u_{\rm m}$ $=$ 1.1 $_{-0.1}^{+0.2}$ $_{-0.2}^{+0.4}$ . The east lobe of DA 240 is indicated to exhibit the lowest value of $u_{\rm e}$ , 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 $\sim\ $ 1 Mpc to those in the smaller objects suggests a possibility that radio galaxies lose their jet power as they evolve from $\sim\ $ 100 kpc to $\sim\ $ 1 Mpc.